10 Must-Watch Future Stars About to Invade MLB

The final month of the 2018 MLB season is right around the corner, and with it comes roster expansion from 25 to 40 players.

The bulk of September call-ups consist of players who have spent the season shuffling between Triple-A and the majors—the fringe middle relievers, spot starters, utility infielders and fifth outfielders of the world.

However, at least a few top prospects always get their first taste of MLB action during this window as well.

Here's a quick look at some of the top prospects who got the call last September, with their placement on the Baseball America midseason top 100 prospects list included for reference:

• 8. OF Victor Robles (WAS)
• 17. RHP Walker Buehler (LAD)
• 18. C Francisco Mejia (CLE)
• 26. 2B Franklin Barreto (OAK)
• 29. C Chance Sisco (BAL)
• 35. OF Alex Verdugo (LAD)
• 43. RHP Brandon Woodruff (MIL)
• 57. RHP Jack Flaherty (STL)
• 68. LHP Anthony Banda (ARI)
• 70. RHP Erick Fedde (WAS)
• 74. OF Willie Calhoun (TEX)
• 76. LHP Luiz Gohara (ATL)
• 91. 1B Ryan McMahon (COL)
• 92. SS J.P. Crawford (PHI)
• 99. OF Austin Hays (BAL)

Which notable prospects are ready to crash the party this time around? Let's take a look.

RHP Sandy Alcantara, Miami Marlins

Michael Reaves/Getty Images

2018 Stats (A+/AAA): 6-3, 3.78 ERA, 1.23 WHIP, 41 BB, 93 K, 121.1 IP

2018 Stats (MLB): 1 GS, W, 5.0 IP, 3 H, 1 ER, 5 BB, 2 K

September Role: Starting rotation

40-Man Roster: Yes

Sandy Alcantara was the prize of the offseason trade that sent Marcell Ozuna to the St. Louis Cardinals in exchange for a package of four prospects. His Miami Marlins debut gave fans plenty of reason to want more.

The 22-year-old made the first big league start of his career on June 29, allowing only three hits and one earned run over five innings against the New York Mets to earn the win. However, an armpit infection sidelined him for nearly a month shortly after that debut.

Once he returned to the mound for a rehab assignment, his velocity was down, so the team opted to keep him in Triple-A.

Manager Don Mattingly recently indicated Alcantara's velocity is back into the upper 90s, according to Andre C. Fernandez of the Miami Herald. As a result, he looks like a safe bet for a September call-up.

As the organization continues on with the rebuilding process, Alcantara and fellow offseason addition Nick Neidert are by far the most promising young arms in the system.

A strong final month for the hard-throwing right-hander could be put him in a prime position to break camp with a spot in the rotation next season. Otherwise, he still has closer upside out of the bullpen thanks to his electric fastball and plus slider. 

3B Michael Chavis, Boston Red Sox

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2018 Stats (A-/AA/AAA): .321/.407/.579, 23 XBH (9 HR), 26 RBI, 36 R

September Role: Platoon partner for 1B Mitch Moreland and 3B Rafael Devers

40-Man Roster: No

After three disappointing seasons following his selection in the first round of the 2014 draft, Michael Chavis broke out in a big way last year, posting a .910 OPS with 31 home runs and 94 RBI at High-A and Double-A.

However, that momentum ground to a halt when he received with an 80-game suspension for a positive performance-enhancing drug test during the offseason.

Since returning to action at the beginning of July, the 23-year-old has been quickly making up for lost time. He has an eye-popping .429/.455/.857 line with three doubles and two home runs in 22 plate appearances since his promotion to Triple-A.

So, where does he fit into Boston's late-season plans? In the short term, he could serve as an upgrade over Eduardo Nunez at third base. Once Rafael Devers returns from a strained hamstring, he'd slide into a platoon role at both corner infield spots.

Both Devers (110 PA, .223/.273/.340) and first baseman Mitch Moreland (93 PA, .235/.290/.376) have struggled against left-handed pitching this season.

Calling up the red-hot Chavis would give the Red Sox another dangerous offensive weapon.

OF Eloy Jimenez, Chicago White Sox

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2018 Stats (AA/AAA): .341/.386/.580, 51 XBH (21 HR), 74 RBI, 62 R

September Role: Starting left fielder

40-Man Roster: Yes

It would appear that the promotion of fellow top prospect Michael Kopech has inspired Eloy Jimenez.

The 21-year-old is hitting .500/.513/.605 with three extra-base hits and five RBI in nine games since Kopech made his MLB debut on Aug. 21.

Then again, he's been raking all season.

Since turning in a breakout 2016 season while he was still a member of the Chicago Cubs organization, Jimenez has quickly become one of the game's elite young talents.

He has arguably the best raw power of any prospect in baseball, but he's more than just an all-or-nothing slugger. He also has a 60-grade hit tool and the on-base skills to develop into a true offensive superstar.

He's already on the 40-man roster, and the White Sox didn't balk at the idea of promoting Kopech for service-time reasons. As such, it's looking increasingly likely that we'll see Jimenez in the majors before the 2018 season is over.

Giving him the everyday left field job for the final month should position him nicely for a spot on next year's Opening Day roster.  

His arrival also would likely provide a healthy spike in attendance for a team that ranks 27th in that category.

C/OF Francisco Mejia, San Diego Padres

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2018 Stats (AAA): .283/.331/.446, 42 XBH (12 HR), 63 RBI, 48 R

2018 Stats (MLB): 0-for-2, 2 BB

September Role: Platoon partner for C Austin Hedges

40-Man Roster: Yes

Prior to being traded to the San Diego Padres in the deal that sent All-Star reliever Brad Hand to the Cleveland Indians, Francisco Mejia was splitting his time between his natural position of catcher and left field.

With Yan Gomes and Roberto Perez forming a solid platoon for Cleveland at the big league level, it was a potential solution to getting Mejia's MLB-ready bat into the lineup.

Since the trade, he's returned to catching full time, and he could get a chance to start pushing Austin Hedges for the starting job in September.

Mejia, 22, established himself as one of the top prospects in baseball with a .342/.382/.514 line between Single-A and High-A during the 2016 season. While he continues to impress at the plate, there are still some questions about his receiving skills.

Pairing him with the defensive-minded Hedges could make for a successful platoon as the Padres get set to usher in a wave of young pitching talent. They could continue to supplement his playing time with occasional starts in left field.

Mejia has already seen big league action in each of the past two seasons. With a strong final month, he could prove he deserves to be up for good.

OF Victor Robles, Washington Nationals

Rich Schultz/Getty Images

2018 Stats (Rk/A-/AAA): .256/.356/.343, 11 XBH (2 HR), 14 RBI, 27 R, 17 SB

September Role: Oft-used fourth outfielder and pinch-runner

40-Man Roster: Yes

Juan Soto has been one of the breakout stars of 2018, hitting .292/.409/.513 with 16 home runs and 50 RBI in 375 plate appearances after starting the season at High-A. But if not for a gruesome hyperextended left elbow that cost Victor Robles three months, he might never have gotten that opportunity.

Robles, 21, has been on the fast track to the majors since he turned in a huge rookie-ball performance in 2014.

He saw his first MLB action last September, when he went 6-for-24 with two doubles and a triple. In the process, he earned himself a spot on the postseason roster.

Given that performance and his strong numbers in the upper levels of the minors, he almost certainly would have gotten the call when Howie Kendrick went down with a ruptured Achilles. Instead, he's spent much of the summer shaking off the rust while Soto has made a name for himself.

While his overall numbers don't jump off the page, Robles is hitting .333/.377/.521 with seven extra-base hits in his last 11 games.

With Bryce Harper potentially headed elsewhere in free agency, Robles could be auditioning for a starting role alongside Soto and Adam Eaton next season. Finding a way to get him semi-regular playing time down the stretch would seem to be in the Nationals' best interest.

LHP Justus Sheffield, New York Yankees

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2018 Stats (AA/AAA): 6-6, 2.56 ERA, 1.17 WHIP, 50 BB, 119 K, 112.2 IP

September Role: Multi-inning reliever

40-Man Roster: No

The New York Yankees have already begun transitioning Justus Sheffield into a relief role in anticipation of him joining the big league bullpen in September.

The 22-year-old was excellent in his role as a starter. He went 6-6 with a 2.48 ERA, 1.17 WHIP and 116 strikeouts in 108.2 innings, and he held opponents to a .196 batting average.

However, his first two appearances out of the bullpen have not gone as smoothly:

• Aug. 21: 2.0 IP, 2 H, 1 ER, 0 BB, 2 K
• Aug. 25: 2.0 IP, 3 H, 1 ER, 0 BB, 1 K

A scout on hand for his last appearance offered up the following observations to Kristie Ackert of the New York Daily News: "His pitches/approach were same as when starting, they didn't play up at all. Lefties weren't fooled at all, meaning everything was squared up, including a long home run by the eight-hole hitter. ... For me, there's gonna be an adjustment period, There's no telling how long it will take."

He's still capable of being a real weapon in New York. It just might take another appearance or two to iron out some issues.

The Yankees don't necessarily need to call him up on Sept. 1. Having him make a few more appearances at Triple-A to settle into his new role before he joins the big league squad may be the prudent approach.

The left-hander has a future in the starting rotation, but if he's going to make an impact in 2018, it will be out of the bullpen.

LF Christin Stewart, Detroit Tigers

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2018 Stats (Rk/AAA): .257/.359/.483, 48 XBH (24 HR), 78 RBI, 67 R

September Role: Starting left fielder

40-Man Roster: No

The Detroit Tigers selected Christin Stewart with the No. 34 overall pick in the 2015 draft after he turned in a stellar junior season at the University of Tennessee.

He's steadily climbed the organizational ranks since beginning his pro career, posting an .863 OPS with 92 home runs in parts of four minor league seasons. His improved approach at the plate now has him looking ready for the majors.

While making the move from Double-A to Triple-A this season, he's simultaneously improved his walk rate (10.1 to 12.9 percent) and lowered his strikeout rate (24.9 to 20.6 percent).

With improved on-base skills and 30-homer potential, Stewart has a chance to be a central figure in the Tigers' rebuilding plans.

He'll be limited to left field defensively, where Detroit can hide his below-average arm and range. He probably fits best in the DH role, which will be vacant once Victor Martinez reaches free agency this offseason.

For now, the final month will give him an opportunity to get his feet wet in a low-pressure situation. That'll help the team decide whether he's ready to take on an everyday role to start the 2019 season.

1B Matt Thaiss, Los Angeles Angels

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2018 Stats (AA/AAA): .281/.336/.472, 57 XBH (16 HR), 74 RBI, 77 R

September Role: Starting first baseman

40-Man Roster: No

Matt Thaiss was arguably the most polished college hitter in the 2016 draft. The Los Angeles Angels selected him with the No. 16 overall pick after he hit .375/.473/.578 with 10 home runs as a junior at the University of Virginia.

His pro career began with a move from catcher to first base. As expected, that put his advanced bat on the fast track to the big leagues.

The 23-year-old made his Triple-A debut on May 22, and he's hit .279/.329/.464 with 23 doubles, 10 home runs and 49 RBI in 380 plate appearances since his early promotion.

He'll never fit the profile of the prototypical power-hitting first baseman, which means he'll need to improve his 7.6 percent walk rate to keep pace offensively at the position. With that said, he has plenty of gap power and the hit tool to be a perennial .280 to .300 hitter, and he's quickly becoming an above-average defender at first base.

With Albert Pujols done for the season following left knee surgery, the door is now wide-open for Thaiss to step into an everyday role for the final month of the season.

If he plays well, it's time for the Angels to start giving serious consideration to cutting Pujols loose.

OF Alex Verdugo, Los Angeles Dodgers

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2018 Stats (AAA): .330/.392/.474, 29 XBH (10 HR), 44 RBI, 44 R

2018 Stats (MLB): .280/.345/.440, 6 XBH (1 HR), 3 RBI, 7 R

September Role: Everyday fourth outfielder

40-Man Roster: Yes

On almost any other team, Alex Verdugo would already have an everyday job at the MLB level.

The 22-year-old is a .310/.367/.444 hitter over five minor league seasons, and he's continued to lay waste to Triple-A pitching this season with a .330/.392/.474 line that includes 29 extra-base hits and only 46 strikeouts in 378 plate appearances.

He'll never be a huge power threat, but his cannon arm and 60-grade hit tool give him a profile similar to Nick Markakis when he was in the prime of his career with the Baltimore Orioles.

There isn't a more polished, MLB-ready hitter in the minors right now. Verdugo has also more than held his own over 56 plate appearances with the big league club.

With Matt Kemp (75 PA, .197 BA, .553 OPS) and Joc Pederson (66 PA, .169 BA, .683 OPS) both slumping badly in August, the door is wide-open for Verdugo to step into a prominent role for the stretch run.

Looking ahead to next season, the Dodgers could move Chris Taylor to second base to open up an everyday spot in the outfield for Verdugo.

He's proved he's ready for the opportunity.

RHP Taylor Widener, Arizona Diamondbacks

2018 Stats (AA): 5-7, 2.68 ERA, 1.00 WHIP, 40 BB, 171 K, 134.1 IP

September Role: Multi-inning reliever and spot starter

40-Man Roster: No

It cost the New York Yankees a pair of prospects—second baseman Nick Solak and right-hander Taylor Widener—to acquire utility man Brandon Drury from the Arizona Diamondbacks during the offseason.

Five months later, they traded Drury to the Toronto Blue Jays for rental starter J.A. Happ.

While Solak has a chance to be a solid player, the loss of Widener could come back to haunt them in the not-too-distant future.

A 12th-round pick in 2016, Widener was used primarily as a reliever during his three seasons at the University of South Carolina and in his pro debut, but his strong three-pitch mix made him a prime candidate to move into a starting role.

He did just that while pitching for the Yankees' High-A affiliate last season, going 7-8 with a 3.39 ERA, 1.15 WHIP and 129 strikeouts in 119.1 innings spanning 27 starts.

The 23-year-old has continued to show swing-and miss-stuff this season thanks to a mid-90s fastball that he backs with a plus slider and a developing changeup. That arsenal has helped him rack up 171 strikeouts—tied with Josh James (HOU) and Conner Menez (SF) for the MiLB lead—and has him knocking on the door for a promotion.

While it's looking more and more like his future will be in the big league rotation, Widener's experience pitching out of the bullpen will serve him well. Unless injury strikes, he figures to join the relief corps, provided he gets the call.

All stats courtesy of Baseball Reference and FanGraphs, unless otherwise noted.