If it feels to you as though there are more rookies in Major League Baseball than usual, you're not wrong. Between the rules allowing for expanded rosters this year and the need to restock those rosters because of positive COVID-19 tests as well as higher injury rates than normal, teams already have used 231 pitchers and 133 position players who meet the qualifications to be a rookie -- that is, they entered the season with fewer than 130 career at-bats, 50 career innings pitched or 45 career days of service time prior to Sept. 1 in a given season.
By comparison, teams last year used 270 pitchers and 171 position players who met the qualifications, albeit over a period of games more than three times as long; the pitching total was the highest of at least the past 10 seasons, while the position player total wasn't far off from the decade's high of 181, set in 2011.
Some of these rookies have been standouts, though with about 80% of the 60-game schedule played, the sample sizes are still small, and the separation between the good ones and the very best is narrow. What follows here is one person's attempt to identify the most impactful rookies this year, split between position players and pitchers rather than by league.
For this exercise, I'm guided by but not limited to the FanGraphs version of wins above replacement, and mindful of sample sizes; while some rookies have excelled while getting fewer than 100 plate appearances or throwing fewer than 30 innings, the larger bodies of work carry more weight as far as I'm concerned.
Hitters
1. OF Kyle Lewis, Seattle Mariners
Lewis tore his right ACL as well as his medial and lateral menisci just six weeks after being taken with the 11th pick in the 2016 draft, and he played just 135 games over the next two seasons. Finally healthy, he reached the majors late last year, homered in each of his first three games and six of his first 10, and began this year with a bang as well, with homers in the Mariners' first two contests.
Perhaps the most promising aspect of the 25-year-old rookie's campaign is his improved plate discipline relative to last year's 75-plate-appearance cup of coffee. He has cut his rate of chasing pitches outside the strike zone from 36.0% to 24.7% and more than tripled his walk rate (from 4.0% to 13.9%) while pulling his strikeout rate out of the red zone (from 38.7% to 26.7%). Oh, and he can play defense, too, as Monday's robbery of a potential grand slam by Oakland's Ramon Laureano illustrated.
2. CF Luis Robert, Chicago White Sox
Ranked seventh on FanGraphs' Top 100 Prospects list entering the year and No. 5 on that of Kiley McDaniel's 2020 list for ESPN, Robert has been as good as advertised. Not only has he cracked the American League's top 10 in home runs and fWAR, but he has provided elite defense in center field, as confirmed by both the highlights and the metrics. He leads all AL center fielders in ultimate zone rating (4.6), all major league center fielders in defensive runs saved (12), and all defenders in Statcast's outs above average (7). His 31.1% strikeout rate, nearly 4-1 strikeout-to-walk ratio and 47.2% rate of swinging at pitches outside the strike zone are causes for some concern but also issues that were anticipated. That he's doing all of this while playing a premium defensive position on a playoff contender at age 23 will probably help him nab the AL Rookie of the Year award, though the gap in more easily measured offense is what puts him behind Lewis in this scribe's view.
3. 2B Jake Cronenworth, San Diego Padres
Acquired from the Rays alongside Tommy Pham last December, Cronenworth is a 26-year-old late bloomer whose skill set includes a passable fastball/slider combo on the mound that has expanded his repertoire beyond that of the usual big league utilityman; he pitched at the University of Michigan and made seven appearances at Triple-A Durham last year but has yet to make a major league appearance. What he has done is play every infield position for the Padres (OK, just one inning at third base), most notably shoring up second base as necessitated by Jurickson Profar's struggles and subsequent move to left field (to fill in for the injured Pham). Via Statcast, his combined total of five outs above average is tied for second in the majors among all infielders, one behind Nolan Arenado. Thanks to the Padres' high visibility in the playoff race, he has become the odds-on favorite to win National League Rookie of the Year honors.
4. C Sean Murphy, Oakland Athletics
A third-round 2016 pick out of Wright State University, Murphy arrived last September and instantly put his claim on a share of the A's catching job. This year, he has taken over regular duty and emerged as one of the league's top backstops. His 137 wRC+ is tied for fourth among all catchers, and it leads all Oakland hitters, helping to offset the underperformance -- and now loss for the season -- of Matt Chapman. Meanwhile, he has been slightly above average in pitch framing and throwing across multiple metrics.
5. SS Andres Gimenez, New York Mets
With a strong second half last year in his age-23 season, Amed Rosario appeared to have solidified his hold on the Mets' shortstop job, but his slow start and underwhelming performance this year (72 wRC+, 0.1 fWAR) has opened the door for the just-turned-22-year-old Giménez, who has outplayed him on both sides of the ball while making 15 starts at shortstop, four at second base and three at third. Though he doesn't have the power or plate discipline (note his 4.8% walk rate) to be a true impact player, his seven steals in eight attempts leads the moribund Mets and ranks among the NL's top 10.
6. LF Ryan Mountcastle, Baltimore Orioles
I caved on my intention to use a 100-plate-appearance cutoff for position players when it came to Mountcastle, whose offensive numbers have been so eye-opening. Through Monday, he was the only rookie with at least 60 PAs to slug over .600, and he ranked fifth in fWAR and seventh in total bases among rookies despite ranking 21st in plate appearances. Despite a reputation for being a free swinger in the minors (4.3% walk rate last year, 4.5% career), he's at 9.2% thus far, though his 39.9% rate of swinging at pitches outside the zone probably won't support such a high rate over a larger sample. Likewise when it comes to his overall level of offensive production given his meager 86.0 mph average exit velocity, to say nothing of the questions about his eventual defensive home, though with Chris Davis back on the injured list, he deserves a long look at first base.
Honorable mentions: 3B Alec Bohm (Phillies) and SS/3B Willi Castro (Tigers), two hot hitters whose defensive metrics dragged them down relative to the others above.
Pitchers
For this exercise, I'm using FanGraphs' 50/50 blend of fielding independent pitching-based WAR and RA9-based WAR (for more on that, see here).
1. RHP Sixto Sanchez, Miami Marlins
He has made just five starts, but the 22-year-old Sanchez has been dominant while showing off electrifying stuff, including a four-seam fastball that can top 100 mph and plus-plus changeup. He was acquired from the Phillies as part of the J.T. Realmuto trade, and he entered the year as a top-20 prospect, albeit with just 18 starts at Double-A under his belt. Thanks to his strong work since debuting on Aug. 22, including a seven-inning complete-game victory against his former team this past Sunday, he has emerged as the staff ace of a Marlins team that's in the NL East race and likely playoff-bound.
2. RHP Devin Williams, Milwaukee Brewers
The one exception to my 30-inning minimum threshold is the owner of the most unhittable pitch in baseball. A 2013 second-round pick, Williams was waylaid in the minors by Tommy John surgery and subpar command, but since moving to the bullpen, he's been able to pump his fastball as high as 100 mph, pairing it with an 85 mph changeup against which batters have collected just one hit -- a Kolten Wong single on Monday -- on his first 48 at-bats ending with the pitch. That lethal 1-2 punch has helped him strike out 52.6% of the batters he has faced, tops for any pitcher in baseball with at least 10 innings.
3. RHP Tony Gonsolin, Los Angeles Dodgers
Lanky redhead Dustin May, with his stiff-legged delivery and his filthy sinker, have gotten more attention, but the less heralded Gonsolin, who at times has had to battle for his roster spot, has been the more effective of the two Dodger rookie starters. A shaggy 26-year-old righty who bears a resemblance to Frank Zappa, he passed the 30-inning threshold only on Tuesday night, his first outing in which he reached 90 pitches -- a stellar seven-inning turn against the Padres. His filthy fastball/splitter/slider/curve combo has befuddled hitters, generating a team-high 14.6% swinging strike rate while helping him limit walks and homers.
4. LHP Justus Sheffield, Seattle Mariners
A 2014 first-round pick who has since been traded by the Indians and Yankees, Sheffield had brief major league stints in each of the past two seasons but has finally settled in this year. While he hasn't been dominant, the 24-year-old sinker/slider southpaw has struck out nearly a batter per frame while holding opponents to just two home runs in 44⅓ innings thanks to a slew of ground balls.
Honorable mentions, starting pitchers: RHP Randy Dobnak, Twins; RHP Cristian Javier, Astros; LHP Kwang Hyun Kim, Cardinals; LHP Jesus Luzardo, Athletics; RHP Dustin May, Dodgers.
Honorable mentions, relievers: RHP James Karinchak, Indians; RHP Jonathan Hernandez, Rangers.