SI:AM | Mason Miller Is MLB’s Most Fearsome Reliever

Anyone who’s ever started a new job knows the feeling. You’re new to the area, perhaps you don’t know the workspace layout very well. Coworkers’ names prove elusive. Maybe you do something mildly embarrassing to leave a shaky early impression—for example, in the case of Baltimore Orioles outfielder Colton Cowser, throw away a treasured keepsake of your nine-time All-Star closer.

There’s a reason they call them rookie mistakes: It’s because rookies make them. A lot of them.

But fear not, newbies, because the 2024 season is now more than a month old, giving plenty of opportunities to shake off the early butterflies and settle into your new lives as big leaguers. To mark the occasion, we present to you the first edition of Sports Illustrated's Rookie Roundup. Each week, we’ll check in on the latest from the game’s standout first-year players, alternating between the American and National Leagues.

The idea is to shine a spotlight on the best (and most entertaining) happenings from rookies far and wide. We’ll cover the headliners, of course—and with the steady influx of young talent, there are plenty of already big names to track—but also aim to uncover hidden gems making names for themselves as the season progresses.

This week’s focus is simple: We’ll highlight the five best AL rookies so far, with the NL’s top five coming next week. All of the players considered have, of course, made their fair share of rookie blunders. But they’ve made up for them (and then some) with their stellar play to this point.

1. Colton Cowser, OF, Baltimore Orioles

Baltimore’s organizational rebuild (read: tank-a-thon) from the late-2010’s resulted in the franchise picking in the top-five of the draft in four consecutive years from ’19 to ’22. One of those picks turned into Cowser, whose 26-game debut last season (with a .115 batting average and zero home runs) left a lot to be desired. Named to the Opening Day roster by manager Brandon Hyde after posting a 1.136 OPS during spring training, Cowser seized an everyday role by the season’s second week and hasn’t looked back.

Cowser’s hitting profile is a modern twist on the "three true outcomes" template: lots of walks and strikeouts, sure, but plenty of hard contact (rather than merely home runs). His 34.3% strikeout rate is the eighth-highest among players with at least 90 plate appearances, and his 38.1% whiff rate is the seventh-highest. But a quick glance at his Baseball Savant page shows plenty of red bars, as Cowser’s barrel rate, hard-hit rate and expected slugging percentage all rank in the 90th percentile or better. Combine all that with a 10.1% walk rate and excellent defense, and Cowser could follow in Gunnar Henderson’s footsteps and make it back-to-back Rookie of the Year awards for the Orioles.

2. Mason Miller, RP, Oakland Athletics

If they gave trophies to teams for putting together five-and-a-half weeks of mediocre play when almost everybody in the baseball world expected nothing short of total ineptitude, the Athletics would get one. The franchise is in an abysmal position, with an owner resolved to move the team to Las Vegas and coming off consecutive 100-plus loss seasons. While the roster is low on household names (an A’s staple as old as time itself), the team has outperformed projections to this point with a 17–18 mark, and Miller’s been a key reason.

A five-year college career spent at Division III Waynesburg (with one season at Gardner-Webb) didn’t keep Miller from being drafted by Oakland in the third round in 2021. He logged just 28 2/3 minor league innings from ’21 to ’23 before debuting in April of last year. Miller pitched in 10 games for the A’s last season—six of them starts—and impressed enough to make the roster out of spring training this season as a part of the bullpen.

That’s an extremely truncated acceleration timeline, which is sort of fitting given how quickly Miller has ascended into becoming the league’s most dominant closer. His fastball averages (yes, averages) 100.7 mph, and he’s struck out 29 of the 54 batters he’s faced this season. Miller is a perfect 8-for-8 in save chances, helping Oakland to a 7–5 record in one-run games. His FIP currently sits at -0.06, and while it’s sure to climb out of the red eventually, Miller’s already established himself as one of baseball’s most electric pitchers.

3. Wilyer Abreu, OF, Boston Red Sox

Abreu earned himself a late August call-up in 2023 but retained his rookie status coming into ‘24. He hit well in his debut, and has built on last year’s showing to quickly become one of Boston’s most important pieces so far.

Abreu ranks fourth among AL rookies in runs scored (16) and RBIs (13), adding strong defense in right field and speed on the base paths, going a perfect 5-for-5 in stolen base attempts. If there’s a reason to temper optimism a bit, it’s the fact that his .297/.381/.473 slash line is buoyed by a .403 BABIP. Statcast paints a far less enthusiastic view of Abreu, with an expected batting average of .225. Even if regression is on the way, he’s shown enough to solidify promise as a bonafide everyday player.

4. Luis Gil, SP, New York Yankees

New York’s rotation has collectively risen to the occasion in reigning Cy Young Award winner Gerrit Cole’s absence, and Gil has more than done his part in the efforts.

The 25-year-old has allowed two runs or fewer in four of his six starts. His most impressive outing was also his most recent: 6 1/3 shutout innings against the Orioles on Wednesday. Gil averages 11.61 strikeouts per nine innings, fourth-most among pitchers who have thrown at least 30 frames. Walks have been an issue, though one that’s trending in the right direction: He gave up 17 free passes in 19 2/3 innings in his first four outings, but has allowed only three in his last two starts.

5. James McArthur, RP, Kansas City Royals

A Royals team coming off of a 106-loss campaign has been one of 2024’s biggest surprises so far, with a 20–15 record and the second-highest run differential (plus-44) in the AL. There’s plenty of credit to go around the roster, but McArthur’s emergence as a lockdown closer (Sunday’s blown save against the Rangers notwithstanding) deserves recognition.

McArthur’s best asset is his control. He’s walked only two of the 66 batters he’s faced so far, compared to 18 strikeouts. Missing bats is also a strength, as McArthur has a whiff rate of 34.9%. Sunday’s meltdown was his first blown save since taking over as the team’s closer, but even then, his strengths were on display. He drew 18 swings on his 30 pitches—with eight of them whiffs—and he didn’t walk a batter. McArthur spent six years in the Philadelphia Phillies’ system working primarily as a starter before being traded to the Royals in 2023, beginning his transition to the bullpen. He seems to have found his rhythm there, and has so far played a crucial role in one of MLB’s best success stories.