15 Must-Watch Future Stars About to Invade MLB in September

As we get ready to flip the calendar from August to September, the MLB landscape will soon be invaded by a wave of young talent as active rosters expand from 25 to 40 players.

For the most part, September call-ups are made up of fringe big leaguers who have been shuttling between Triple-A and the MLB roster throughout the season.

However, there are always a handful of notable prospects who get a chance to debut as well.

Ahead we'll take a closer look at 15 prospects with future star potential who have a good chance of seeing MLB action over the season's final month.

3 Lesser-Known Prospects Who Could Impact Playoff Races

1B/OF Christian Walker, Arizona Diamondbacks

After bouncing around on the waiver wire from Baltimore to Atlanta to Cincinnati to Arizona during the offseason, Christian Walker is putting together an impressive season for Triple-A Reno.

The 26-year-old is hitting .312/.385/.607 with 34 doubles, 32 home runs and 112 RBI, and while he may not have a clear path to playing time at first base or in the outfield, he could be a dangerous weapon off the bench for a contending D-backs team.

RHP Dillon Maples, Chicago Cubs

The Cubs gave Dillon Maples a $2.5 million bonus as a 14th-round pick in 2011, and he's popped back up on the prospect radar this season after a disappointing start to his pro career.

Command remains the biggest question mark. He's still walking batters at a 5.3 BB/9 clip this year, but he's worked around those free passes to post a 2.31 ERA and tally 13 saves. Armed with a fastball that can touch triple digits and a lethal slider, he's also racked up 99 strikeouts in 62.1 innings out of the bullpen.

He won't be immediately thrown into the fire, but he has the stuff to carve out a high-leverage role down the stretch on a shaky Cubs relief corps.

RHP Rogelio Armenteros, Houston Astros

Cuban right-hander Rogelio Armenteros has quietly been a prospect on the rise for the Astros over the past two seasons, and it sounds like general manager Jeff Luhnow is at least considering a call-up for the 23-year-old, per Jake Kaplan of the Houston Chronicle:

"He's been terrific in Triple A and we do see him as a guy who can help out. Now the question is, 'How many innings will he get?' We're expecting to have (Michael) Feliz and (Will) Harris and (Tony) Sipp and everybody back at that point.

So, with all of those players coming back and the guys we have now also still on the team, it becomes less likely that innings are available. But he's certainly a guy we're going to have to discuss."

Armenteros has posted a 1.97 ERA, 1.03 WHIP and 10.8 K/9 in 118.2 innings between Double-A and Triple-A, working primarily as a starter.

3B Jeimer Candelario, Detroit Tigers

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B/R Top 100 Prospect Rank: Not ranked

Stats (AAA): .263/.342/.485, 104 H, 54 XBH (15 HR), 68 RBI, 51 R

Outlook

Jeimer Candelario finally emerged from an impenetrable infield blockade when the Chicago Cubs traded him to the Detroit Tigers at the deadline in the deal that brought reliever Justin Wilson and catcher Alex Avila to the North Side.

The 23-year-old might not be a superstar in the making, but he has all the tools to emerge as the everyday third baseman for a Tigers team that desperately needs an infusion of young talent.

Candelario carries a 10.7 percent walk rate over the course of his minor league career. Those on-base skills coupled with playable gap power and solid defense should give him a good chance of winning the starting third base gig in 2018.

Incumbent third baseman Nicholas Castellanos has already indicated that he's open to the idea of moving to right field.

3B Brian Anderson, Miami Marlins

Brace Hemmelgarn/Getty Images

B/R Top 100 Prospect Rank: Not ranked

Stats (AA/AAA): .277/.362/.498, 117 H, 46 XBH (22 HR), 80 RBI, 73 R

Outlook

In a Miami Marlins farm system that is almost completely bereft of top-tier position-player talent, third baseman Brian Anderson might be the lone standout.

The 24-year-old posted a .866 OPS with three doubles and five home runs in 22 games in the Arizona Fall League last year, and he's used that performance as a springboard for a breakout season.

Here's what MLB.com had to say about Anderson:

"Anderson's performance in the Fall League set the stage for a 2017 campaign that's seen him improve on all fronts against advanced competition. While there once was concern about whether he would hit for enough average and power to profile as a regular at the hot corner, Anderson has done his part to prove his detractors wrong with a career-best campaign in '17."

Looking ahead to next season, he's blocked by veteran Martin Prado, who has two years and $28.5 million left on the extension he signed last fall.

However, he's currently recovering from knee surgery and could be shopped during the offseason.

1B Ronald Guzman, Texas Rangers

Charlie Riedel/Associated Press

B/R Top 100 Prospect Rank: Honorable Mention

Stats (AAA): .304/.375/.446, 137 H, 37 XBH (12 HR), 61 RBI, 75 R

Outlook

With Mike Napoli headed for free agency at season's end, the door could swing wide open for Ronald Guzman to take over as the Texas Rangers' everyday first baseman in 2018.

He has little left to prove at the minor league level.

The 22-year-old inked a $3.45 million bonus as part of the same 2011 international class that saw Nomar Mazara land a then-record $4.95 million bonus of his own. Those two could soon be hitting side-by-side in the middle of the big league lineup.

While Guzman may never develop into a prototypical home run threat at the first base position, his plus hit tool and gap power will go a long way toward offsetting any perceived offensive shortcomings.

LHP Luiz Gohara, Atlanta Braves

B/R Top 100 Prospect Rank: 95

Stats (A+/AA/AAA): 6-4, 2.73 ERA, 1.22 WHIP, 140 K, 118.2 IP

Outlook

In an Atlanta Braves system loaded with high-end pitching talent, left-hander Luiz Gohara has quickly risen through the organizational ranks this season.

The burly 6'3" southpaw was acquired from the Seattle Mariners during the offseason in exchange for outfielder Mallex Smith and reliever Shae Simmons.

Already a solid prospect, he's seen his stuff tick up this season, per MLB.com: "With better conditioning came more strength, and Gohara's velocity jumped up to the mid-90s with ease and even approached triple digits. His slider tightened and got better as well, serving as a strikeout pitch while also continuing to baffle left-handers."

Other pitching prospects like Kolby Allard, Mike Soroka, Kyle Wright and Ian Anderson might have a higher ceiling long-term, but expect Gohara to be the first from that top-tier group to arrive in the majors.

A strong September audition could put him in position to break camp with a rotation spot next spring.

SS J.P. Crawford, Philadelphia Phillies

Justin K. Aller/Getty Images

B/R Top 100 Prospect Rank: 87

Stats (AAA): .237/.348/.395, 107 H, 37 XBH (14 HR), 57 RBI, 72 R

Outlook

The Philadelphia Phillies are still waiting for the arrival of shortstop J.P. Crawford, who has long been expected to be a cornerstone of the club's rebuilding process.

The 22-year-old scuffled to a .250/.349/.339 line in his first extended action in the upper minors last season, and it was more of the same for a good portion of the 2017 season.

He has picked up his production of late, though.

• April-June: 291 PA, .203/.321/.276, 12 XBH
• July-Aug: 239 PA, .278/.381/.537, 25 XBH

It sounds like that will be enough for him to be among the team's call-ups in September. The hope is that he takes the shortstop job and runs with it.

"[Manager Pete] Mackanin said he is not certain which players will be promoted once rosters expand, but Triple-A shortstop J.P. Crawford ... is certain to be one of them," wrote Todd Zolecki of MLB.com.

2B/OF Willie Calhoun, Texas Rangers

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B/R Top 100 Prospect Rank: 75

Stats (AAA): .294/.351/.564, 136 H, 61 XBH (29 HR), 83 RBI, 75 R

Outlook

His future defensively is still up in the air, but there's no question Willie Calhoun has the bat to make an impact at the next level.

The 22-year-old is a .283/.347/.516 hitter in parts of three minor league seasons since going in the fourth round of the 2015 draft out of JUCO powerhouse Yavapai College.

He was the centerpiece of the July trade that sent Yu Darvish to the Los Angeles Dodgers, and now he looks poised to make his MLB debut in a Texas Rangers uniform this September.

He'll likely move off second base and shift to either left field or designated hitter at the next level, but it's his bat that will drive his value.

LHP Stephen Gonsalves, Minnesota Twins

Brace Hemmelgarn/Getty Images

B/R Top 100 Prospect Rank: 69

Stats (AA/AAA): 9-4, 2.96 ERA, 1.09 WHIP, 118 K, 109.1 IP

Outlook

The Minnesota Twins opted against promoting Stephen Gonsalves when they needed a fill-in starter on Aug. 21, turning instead to veteran journeyman Tim Melville.

Does that mean they'll also balk at the idea of a September call-up?

One thing working in his favor is the fact that he'll need to be added to the 40-man roster this offseason regardless in order to be protected from the Rule 5 draft.

The 23-year-old also has room to tack some innings on to his season total after missing time in April recovering from a shoulder strain. His polished four-pitch mix is ready to be tested at the next level.

He could be an immediate rotation upgrade over Dillon Gee for the stretch run or work out of the bullpen to get his feet wet before competing for a starting spot next spring.

IF Ryan McMahon, Colorado Rockies

Wilfredo Lee/Associated Press

B/R Top 100 Prospect Rank: 62

Stats (AA/AAA): .351/.397/.571, 163 H, 61 XBH (19 HR), 85 RBI, 72 R

Outlook

Ryan McMahon saw his prospect star fade a bit last year when he posted a lackluster .242/.325/.399 line in a full season with Double-A Hartford.

The 22-year-old has absolutely raked against upper-level pitching this season, though.

He made his MLB debut on Aug. 12 and went 2-for-9 with a double in a brief four-game stint in the majors before returning to the minors.

That makes him a no-brainer to be recalled once rosters expand. He could provide some useful pop in a utility role down the stretch before perhaps competing for the starting first base job next spring.

"I have every reason to believe he'll mash here when he gets his chance," Rockies assistant hitting coach Jeff Salazar told Max Gelman of MLB.com.

RHP Erick Fedde, Washington Nationals

Nam Y. Huh/Associated Press

B/R Top 100 Prospect Rank: 58

Stats (AA/AAA): 4-5, 3.69 ERA, 1.16 WHIP, 79 K, 90.1 IP

Outlook

Erick Fedde became the top pitching prospect in the Washington Nationals system after the club traded Lucas Giolito, Reynaldo Lopez and Dane Dunning to acquire Adam Eaton from the Chicago White Sox during the winter meetings.

The 24-year-old slipped to the No. 18 pick in the 2014 draft after undergoing Tommy John surgery in May of his junior season at UNLV. That roll of the dice has paid off for the Nats.

While he might not have ace upside, he does have a high floor and a great chance to at least develop into a solid middle-of-the-rotation arm.

His first taste of the big leagues came in July when he posted a 9.39 ERA in three starts filling in for Stephen Strasburg. An inauspicious start to say the least, but no reason to sour on his long-term potential in the rotation.

For now, he'll be a useful multi-inning arm out of the bullpen and the next man up if injury strikes in the rotation again.

RHP Jack Flaherty, St. Louis Cardinals

Brace Hemmelgarn/Getty Images

B/R Top 100 Prospect Rank: 56

Stats (AA/AAA): 14-4, 2.18 ERA, 1.04 WHIP, 147 K, 148.2 IP

Outlook

Jack Flaherty entered the season a notch below Alex Reyes and Luke Weaver among St. Louis Cardinals pitching prospects.

However, with a standout performance in the upper minors, he's established himself as a top-tier prospect in his own right.

With Michael Wacha (7.25 ERA) and Adam Wainwright (7.36 ERA) both struggling in August and Mike Leake traded to the Seattle Mariners, per The Athletic's Ken Rosenthal, he could give the rotation a real shot in the arm.

His first test will come on Friday, as the team announced shortly after the Leake trade that Flaherty would be called up to start against the San Francisco Giants.

The 21-year-old has already turned in a career-high workload with 148.2 innings, so the team might not want to push him too much further. Still, he should see at least a handful of starts if Friday goes well.

RHP Chance Adams, New York Yankees

B/R Top 100 Prospect Rank: 46

Stats (AA/AAA): 15-4, 2.24 ERA, 1.05 WHIP, 129 K, 144.2 IP

Outlook

Chance Adams was not even ranked among the top 30 prospects in the New York Yankees system heading into the 2016 season, per Baseball America.

Now he's arguably the top pitching prospect in their loaded system after turning in a breakout season last year (13-1, 2.33 ERA, 0.90 WHIP) and building off that impressive performance with more of the same in the upper minors.

The 23-year-old has made strides with his off-speed stuff in support of a mid-90s fastball, as MLB.com explained: 

"His heater doesn't feature a lot of life, but his combination of velocity and command makes it tough to hit. His slider has gotten harder and sharper in pro ball, arriving in the mid-80s and giving him a second well above-average pitch at times.

Adams' fading changeup has also blossomed as he has used it more, and his curveball ranks as his worst pitch yet still grades as average. He locates his pitches well and loves to attack hitters."

Sounds like a recipe for success, though he may have to settle for a relief role to begin his pro career since the Yankees' rotation has performed well of late.

2B/SS Franklin Barreto, Oakland Athletics

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B/R Top 100 Prospect Rank: 44

Stats (AAA): .285/.333/.453, 133 H, 41 XBH (15 HR), 53 RBI, 63 R

Outlook

Franklin Barreto is once again putting up impressive numbers in what has been an up-and-down season in Triple-A.

• April: 100 PA, .333 BA, .934 OPS, 8 XBH
• May-July: 338 PA, .244 BA, .684 OPS, 24 XBH
• August: 113 PA, .330 BA, .904 OPS, 12 XBH

The 21-year-old was the key prospect acquired in the deal that sent Josh Donaldson to the Toronto Blue Jays, and he still has a chance to salvage that deal on the Oakland side of things.

All signs point to Barreto replacing veteran Jed Lowrie as the starting second baseman at some point next season. He'll join Marcus Semien to form a promising young middle infield tandem.

An extended audition this September would go a long way in helping the A's decide whether to exercise a $6 million option on Lowrie, who could also be a trade candidate.

SS Willy Adames, Tampa Bay Rays

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B/R Top 100 Prospect Rank: 24

Stats (AAA): .272/.359/.406, 130 H, 41 XBH (9 HR), 55 RBI, 69 R

Outlook

Tampa Bay Rays prospect Willy Adames—just like the aforementioned Franklin Barreto—was the centerpiece of a blockbuster deadline trade when he was still a low-level minor leaguer.

The 21-year-old was acquired in the 2014 deal in which David Price was shipped to the Detroit Tigers, and he's steadily climbed the organizational ladder as the shortstop of the future.

Here's what MLB.com wrote about him:

"To go along with loud tools and both sides of the ball, Adames continues to receive raves from club officials for his plus makeup and leadership skills. He has all the ingredients to develop into an All-Star-caliber shortstop capable of hitting in the middle of a lineup and could soon be ready for his first taste of the Majors."

He's still working to turn his raw power into in-game production, but he has the hit tool and defensive skills to make an immediate impact in the majors.

Light-hitting Adeiny Hechavarria is manning shortstop, and he's under team control through next season. However, with a $4.35 million salary this season and a moderate raise likely coming his way in arbitration, he could be a non-tender candidate.

RHP Brent Honeywell, Tampa Bay Rays

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B/R Top 100 Prospect Rank: 14

Stats (AA/AAA): 12-9, 3.63 ERA, 1.26 WHIP, 167 K, 131.1 IP

Outlook

What kind of impact can Brent Honeywell have for the Tampa Bay Rays down the stretch?

Jason Martinez of MLBTradeRumors offered up an intriguing best-case scenario. 

"Not only is Honeywell one of the top pitching prospects in baseball, there’s also a chance that he can help the Rays down the stretch in the same way that rookie David Price did in 2008 when he pitched 14 innings in September and another 5 2/3 frames in the playoffs for the AL champs," Martinez wrote. 

The 22-year-old might be the most MLB-ready pitching prospect in the game thanks to a polished five-pitch repertoire that includes a mid-90s fastball, plus changeup and a screwball that gives opposing hitters an unfamiliar look.

However, he was suspended for four games Monday for what the Rays called "disciplinary reasons," and they are quickly slipping out of contention. They could opt against tacking too many more innings onto his arm this year. 

RHP Walker Buehler, Los Angeles Dodgers

B/R Top 100 Prospect Rank: 12

Stats (A+/AA/AAA): 3-3, 3.22 ERA, 1.10 WHIP, 123 K, 86.2 IP

Outlook

As if a 91-win Los Angeles Dodgers team needed further reinforcements.

Walker Buehler has been one of the fastest-rising prospects in baseball this season, as he's delivered on the potential that made him the No. 24 pick in the 2015 draft even with Tommy John surgery looming.

The 23-year-old was moved to the bullpen in early August to limit his innings in anticipation of a September call-up, and he could be an X-factor out of the bullpen as the Dodgers chase a title.

He has the power stuff to blow hitters away in short stints and a deep enough repertoire to be a multi-inning weapon as well, as MLB.com noted on his prospect bio:

"Buehler usually operated with a 90-96 mph fastball in college, and he opened eyes by throwing in the mid-90s during his brief pro debut and hitting 99 during instructional league. He has maintained that velocity as a starter on tight pitch counts this year, and he backs up his heat with an array of impressive secondary pitches. He can miss bats with both his curveball and slider (and morph the latter pitch into a harder cutter), and his changeup shows flashes of becoming an above-average offering."

He'll have a chance to get his feet wet in a relatively low-pressure situation, as the Dodgers have all but wrapped up the NL West title. He could pitch his way onto the postseason roster.

All stats courtesy of Baseball Reference and current through Tuesday's games.