The 1 Rookie Who Could Make or Break Each MLB Contender
The value of experience can't be overstated.
Playing in a pennant race, when every game is crucial and the lights shine brightest, is simply a different animal than your run-of-the-mill regular-season game.
That experience has to start somewhere, and for each MLB contender, there's at least one rookie on the roster who is in a position to make a real difference.
Ahead, we've taken a closer look at the one rookie on each roster who figures to make the biggest impact the rest of the way and on into October if his team reaches the postseason.
While performance to date played a part in deciding which rookie got the nod for each team, the biggest factor was a player's expected role for the stretch run. After all, it's hard to make an impact if you're watching from the bench.
Note: To be considered a contender, teams needed to have at least a 10 percent chance of reaching the postseason, according to the latest odds from FanGraphs.
Arizona Diamondbacks: RP Yoshihisa Hirano
1 of 13
2018 Stats: 66 G, 31 HLD, 1.98 ERA, 1.03 WHIP, 19 BB, 50 K, 59.0 IP, 1.4 WAR
Role: Setup reliever
Outlook
After 11 seasons and 156 saves with the Orix Buffaloes, reliever Yoshihisa Hirano signed a two-year, $6 million deal with the Arizona Diamondbacks.
That's quietly been one of the best signings of the offseason.
The 34-year-old has used his fastball/splitter combination to befuddle opposing hitters all season while pitching alongside Archie Bradley and Brad Boxberger at the back of the bullpen. He doesn't have eye-popping strikeout numbers, but he's held opposing hitters to a .202 batting average and induced a solid 49.1 percent groundball rate.
With Bradley (20 G, 7.36 ERA) and Boxberger (14 G, 6.00 ERA) both struggling since the All-Star break, Hirano (20 G, 1.50 ERA) has become the Diamondbacks' most reliable late-inning option.
Other Rookies on the Active Roster
- SP/RP Matt Koch
- RP Yoan Lopez
- RP Jimmie Sherfy
- IF Ildemaro Vargas
- 1B/OF Christian Walker
Atlanta Braves: LF Ronald Acuna Jr.
2 of 13
2018 Stats: 377 PA, .289/.355/.563, 46 XBH (23 HR), 50 RBI, 60 R, 3.8 WAR
Role: Starting left fielder
Outlook
Even after missing a month with a left knee injury, Ronald Acuna Jr. is one of the leading contenders for NL Rookie of the Year honors.
The 20-year-old phenom established himself as the consensus top prospect in baseball by hitting .325/.374/.522 with 60 extra-base hits and 44 stolen bases over three minor league levels last season, and he's continued to show superstar potential in Atlanta.
He just wrapped up a monster August that saw him hit .336/.405/.698 with 11 home runs and 21 RBI in 131 plate appearances. It's no coincidence that the Atlanta Braves seized control of the NL East race at the same time he was swinging a hot bat.
Left-hander A.J. Minter deserves a nod here as well. He's converted 10 of 12 save chances with a 3.32 ERA and 11.4 K/9 since taking over the closer job at the beginning of July, and he figures to continue on in that role the rest of the way.
Other Rookies on the Active Roster
- RP Kolby Allard
- RP Jesse Biddle
- RP Shane Carle
- RP A.J. Minter
- SP Touki Toussaint
- RP Bryse Wilson
- RP Dan Winkler
- RP Kyle Wright
Boston Red Sox: SP Brian Johnson
3 of 13
2018 Stats: 34 G, 12 GS, 4-4, 4.36 ERA, 1.44 WHIP, 30 BB, 80 K, 88.2 IP, 1.1 WAR
Role: No. 5 starter or middle reliever
Outlook
Brian Johnson was the No. 31 overall pick in the 2012 draft, and he ranked among the top 10 prospects in the Boston Red Sox organization three years running from 2015 to 2017, according to Baseball America.
However, his prospect star had begun to fade as he entered his age-27 season and had still not broken through at the MLB level.
Out of minor league options, Johnson spent the first three months of the season pitching primarily out of the bullpen before injuries opened up a spot for him in the starting rotation at the end of June.
He's gone 3-2 with a 4.27 ERA in 11 starts and one relief appearance since June 28.
If the Red Sox advance to the ALCS, there's an outside chance he could get the start in Game 4. Otherwise, he'll be a valuable weapon out of the bullpen, as he's held lefties to a .225 average and .648 OPS in 97 plate appearances.
Other Rookies on the Active Roster
- RP Ryan Braiser
- RP William Cuevas
- IF Tzu-Wei Lin
- RP Bobby Poyner
- 1B Sam Travis
- SP/RP Hector Velazquez
Chicago Cubs: 3B David Bote
4 of 13
2018 Stats: 157 PA, .248/.325/.445, 14 XBH (6 HR), 26 RBI, 19 R, 1.0 WAR
Role: Part-time third baseman and pinch hitter
Outlook
David Bote began 2018 well off the top prospect radar.
The 25-year-old hit a respectable .272/.353/.438 with 30 doubles, 14 home runs and 59 RBI at Double-A last season and then followed that up with a .931 OPS, four home runs and 14 RBI in 76 plate appearances during the Arizona Fall League.
Yet, he still ranked as just the No. 12 prospect in the Chicago Cubs system heading into the season, according to Baseball America.
After a pair of brief call-ups early in the season, he was promoted for good at the end of June. His walk-off grand slam on Aug. 12 against the Washington Nationals has been one of the highlights of the season for the Cubs, and he's proved capable of making a huge impact offensively when he's swinging a hot bat.
With Jason Heyward now battling a hamstring injury, Kris Bryant has moved to right field since returning from the disabled list and Bote continues to see regular playing time at the hot corner.
Other Rookies on the Active Roster
- C Victor Caratini
- C Taylor Davis
- RP Dillon Maples
- RP James Norwood
- RP Randy Rosario
Cleveland Indians: SP Shane Bieber
5 of 13
2018 Stats: 15 GS, 8-3, 4.66 ERA, 1.37 WHIP, 15 BB, 87 K, 85.0 IP, 0.7 WAR
Role: No. 4 starter
Outlook
With Danny Salazar dealing with a bum shoulder and Josh Tomlin demoted to the bullpen, the Cleveland Indians needed someone to shore up the back end of the rotation behind the excellent trio of Corey Kluber, Carlos Carrasco and Mike Clevinger.
Shane Bieber has been that someone.
The 23-year-old rapidly rose up prospect lists with a breakout 2017 season, going 10-5 with a 2.86 ERA, 1.13 WHIP and 162 strikeouts in 173.1 innings over three minor league levels in his first full professional season.
An even better start to the 2018 season earned him a quick promotion, and he's since brought some much-needed stability to what was once an area of uncertainty as the All-Star break neared.
Speedy Greg Allen also continues to see regular playing time in center field with deadline-addition Leonys Martin now out for the season.
Other Rookies on the Active Roster
- CF Greg Allen
- RP Adam Cimber
- C Eric Haase
- SP Adam Plutko
Colorado Rockies: 1B Ryan McMahon
6 of 13
2018 Stats: 184 PA, .238/.315/.396, 15 XBH (5 HR), 19 RBI, 16 R, 0.4 WAR
Role: Part-time first baseman and pinch hitter
Outlook
The Colorado Rockies leaned heavily on a trio of rookies in the starting rotation last season—German Marquez, Kyle Freeland and Antonio Senzatela—on their way to clinching a postseason berth.
A similar impact was expected from Ryan McMahon when he entered spring training as the leading candidate to take over as the starting first baseman.
Early struggles earned him a ticket back to the minors, and he's been up and down for much of the year, but he is finally making an impact at the plate. The 23-year-old has a .276/.354/.483 line that includes three doubles and three home runs in 65 plate appearances since the beginning of August.
High-priced veteran Ian Desmond is still seeing the bulk of the playing time at first base, but McMahon is making a case for at least a platoon role.
Other Rookies on the Active Roster
- RP Yency Almonte
- OF Noel Cuevas
- IF/OF Garrett Hampson
- RP Sam Howard
- RP D.J. Johnson
- C Tom Murphy
- RP Harrison Musgrave
Houston Astros: SP Josh James
7 of 13
2018 Stats: 1 GS, 0-0, 5.40 ERA, 1.20 WHIP, 3 BB, 9 K, 5.0 IP, 0.0 WAR
Role: No. 5 starter or multi-inning reliever
Outlook
After trotting out the same five starting pitchers for most of the season, the Houston Astros have run into some injury issues here in the second half with Charlie Morton (shoulder) and Lance McCullers Jr. (forearm) both landing on the disabled list.
Left-hander Framber Valdez was the first rookie to get the call, and he's pitched extremely well with a 1.26 ERA and 0.91 WHIP in 14.1 innings spanning two starts and one relief appearance.
However, it's hard-throwing Josh James who could make the bigger impact the rest of the way.
James, 25, was absent from the Astros' top-30 prospect list when the season began, according to Baseball America. He's broken out in a big way this season, though.
Armed with a fastball that touches triple digits and a solid slider/changeup mix, he punched out 171 batters in 114.1 innings between Double-A and Triple-A before making his MLB debut on Sept. 1.
With neither rookie likely to make a start in the postseason, James profiles as the better bullpen option thanks to his swing-and-miss stuff.
Other Rookies on the Active Roster
- IF/OF J.D. Davis
- RP Dean Deetz
- RP Cionel Perez
- C Max Stassi
- SP Framber Valdez
Los Angeles Dodgers: SP Walker Buehler
8 of 13
2018 Stats: 18 GS, 6-4, 2.92 ERA, 1.02 WHIP, 28 BB, 115 K, 104.2 IP, 2.5 WAR
Role: No. 2/3 starter
Outlook
The Los Angeles Dodgers have been hit hard by injuries this season, particularly in the starting rotation, as 11 different pitchers have started at least one game.
That's made the performance of rookie Walker Buehler that much more important.
Rather than filling a spot at the back of the staff in his first full season in the majors, he's pitched at the top of the rotation for much of the year. It wouldn't be the least bit surprising to see him on the mound as early as Game 2 in a postseason series.
The 24-year-old has moved quickly since returning from the Tommy John surgery that caused him to slip to No. 24 overall in the 2015 draft and shown no lingering effects. He has all the makings of a future ace and could really make a name for himself this October.
Other Rookies on the Active Roster
- RP JT Chargois (injured)
- C/IF Kyle Farmer
- RP Caleb Ferguson
- C Rocky Gale
- OF Tim Locastro
- OF Alex Verdugo
Milwaukee Brewers: RP Corbin Burnes
9 of 13
2018 Stats: 19 G, 3-0, 1 HLD, 2.88 ERA, 1.08 WHIP, 9 BB, 24 K, 25.0 IP, 0.4 WAR
Role: Multi-inning reliever
Outlook
A strong case can be made that Freddy Peralta has been the most important rookie for the Milwaukee Brewers to this point.
However, with Zach Davies back healthy and Gio Gonzalez acquired from the Washington Nationals, the 22-year-old will now move into a long-relief role down the stretch in an effort to limit what has already been a career-high workload.
Meanwhile, Corbin Burnes has become one of the team's most reliable bullpen options.
The 23-year-old emerged as a leaguewide top-100 prospect last season when he went 8-3 with a 1.67 ERA, 0.95 WHIP and 140 strikeouts in 145.2 innings between High-A and Double-A. While his future may still be in the rotation, he's been used exclusively as a reliever since making his MLB debut on July 10.
He's worked 8.2 scoreless innings in his last five appearances and will continue to be a crucial piece of the relief corps in the heat of a playoff chase.
Other Rookies on the Active Roster
- C Jacob Nottingham
- SP/RP Freddy Peralta
- RP Brandon Woodruff
New York Yankees: 3B Miguel Andujar
10 of 13
2018 Stats: 520 PA, .297/.331/.523, 63 XBH (23 HR), 76 RBI, 74 R, 1.8 WAR
Role: Starting third baseman
Outlook
The battle between Miguel Andujar and Gleyber Torres for AL Rookie of the Year honors will be a compelling one.
Both players are having excellent debut seasons:
A huge month of August has probably put Andujar in the driver's seat, as he hit .320/.344/.623 with 10 home runs and 29 RBI in 30 games while hitting primarily in the cleanup spot in the lineup.
That production has gone a long way toward offsetting the loss of Aaron Judge, and it's made him the most important rookie on the New York Yankees roster. Torres still has the higher long-term ceiling, and he's more than capable of making a major impact in his own right, but Andujar is the guy right now.
Other Rookies on the Active Roster
- C Kyle Higashioka
- RP Jonathan Loaisiga
- RP Stephen Tarpley
- 2B/SS Gleyber Torres
Oakland Athletics: RP Lou Trivino
11 of 13
2018 Stats: 60 G, 8-2, 20 HLD, 2.15 ERA, 1.09 WHIP, 30 BB, 74 K, 67.0 IP, 2.2 WAR
Role: Setup reliever
Outlook
Shohei Ohtani and Forrest Whitley would have been the popular answers during spring training to the question of which AL West rookie pitcher would have the highest WAR.
No one would have said Lou Trivino.
Yet there he is, ranked fifth among all rookie pitchers and first among rookie relievers:
- Dereck Rodriguez, SF: 3.0
- Jack Flaherty, STL: 2.9
- Brad Keller, KC: 2.9
- Walker Buehler, LAD: 2.5
- Lou Trivino, OAK: 2.2
Not bad for a 26-year-old who has never been counted among his team's top 30 prospects, according to Baseball America, let alone received any sort of attention on a national level.
Armed with a heavy sinking fastball that averages 98.5 mph on the radar gun, an excellent cutter and the occasional biting curveball, he's punched out hitters at a 9.9 K/9 clip while racking up 20 holds.
It's surprising contributors like him who have helped the Oakland Athletics emerge as unexpected contenders.
Other Rookies on the Active Roster
- IF Franklin Barreto
- OF Dustin Fowler
- CF Ramon Laureano
- OF Nick Martini
- C Beau Taylor
- RP J.B. Wendelken
Philadelphia Phillies: RP Seranthony Dominguez
12 of 13
2018 Stats: 44 G, 14/18 SV, 11 HLD, 3.04 ERA, 0.93 WHIP, 15 BB, 63 K, 47.1 IP, 1.9 WAR
Role: Part of closer-by-committee
Outlook
The early struggles of Hector Neris (44 G, 10/13 SV, 5.22 ERA) left a glaring hole at the back of the Philadelphia Phillies bullpen.
The surprise emergence of Seranthony Dominguez filled it.
The 23-year-old had worked almost exclusively as a starter up until this season when he got off to a red-hot start with a 1.62 ERA, 0.66 WHIP and a 21-to-3 strikeout-to-walk ratio in 16.2 innings between Double-A and Triple-A.
That earned him his first call-up on May 7, and he quickly pitched his way into the closer's role.
However, he's been struggling of late, posting a 7.45 ERA and 1.45 WHIP with two losses and three blown saves in 11 August appearances.
The Phillies are now employing more of a closer-by-committee with veterans Pat Neshek and Tommy Hunter, but they need Dominguez to get back to pitching at a high level.
Other Rookies on the Active Roster
- C Jorge Alfaro
- RP Victor Arano
- OF Dylan Cozens
- SS J.P. Crawford
- RP Austin Davis
- IF/OF Scott Kingery
- OF Roman Quinn
- RP Yacksel Rios
St. Louis Cardinals: SP Jack Flaherty
13 of 13
2018 Stats: 23 GS, 8-6, 2.83 ERA, 1.05 WHIP, 46 BB, 154 K, 127.1 IP, 2.9 WAR
Role: No. 1/2 starter
Outlook
Center fielder Harrison Bader leads all rookies with 3.9 WAR—thanks in large part to his stellar outfield defense—but that doesn't make him the most important rookie on the St. Louis Cardinals roster.
That title belongs to starter Jack Flaherty.
The 22-year-old has gone 4-0 with a 1.22 ERA and 0.84 WHIP in his last six starts, holding opposing hitters to a .140 average and racking up 10.5 K/9 during that span.
With Carlos Martinez pitching out of the bullpen, Michael Wacha sidelined with a strained oblique and Miles Mikolas regressing since the All-Star break, the Cardinals needed someone to provide a boost to the rotation.
Flaherty has been that guy, and he's now the ace of the staff in St. Louis.
Other Rookies on the Active Roster
- CF Harrison Bader
- OF Adolis Garcia
- SP Austin Gomber
- RP Jordan Hicks
- RP Dakota Hudson
- C Carson Kelly
- IF/OF Yairo Munoz
- RP Daniel Poncedeleon
- LF Tyler O'Neill
- RP Tyler Webb
- 1B/3B Patrick Wisdom
All stats courtesy of Baseball Reference and FanGraphs, and accurate through Sept. 4.
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