While there are still free agents looking for new homes and trade partners plotting further roster shakeups, the time has come for our fridges to sign one-year contracts with brand-new calendars and for us to finally call the 2025 baseball season “this year.”
Now, with a new year upon us, it’s time to look forward and speculate about what lies ahead. For baseball fans, that includes exciting breakouts by players who have yet to hit their performative ceilings, players for whom 2025 will be the banner year in which they establish their place among the best in baseball.
Starting pitcher breakout candidates in 2025
In future articles, I will tackle relievers and position players, but first I will analyze the starting pitchers whose fortunes are due to rise this year. In my analyses, I will be using a project of mine called LASR (League-Adjusted Standardized Rating) which places all stats onto the same 20-80 scouting scale based on league qualifiers. On the scale, 50 is average and 10 points in either direction is one standard deviation (all data is sourced from FanGraphs).
So, without further ado, since baseball is known to be incredibly predictable and prophecies such as these absolutely never look silly in hindsight, it’s time to pronounce the starters who will prove to be undeniable aces in 2025.
Brandon Pfaadt
The Arizona Diamondbacks recently pulled off the biggest surprise of the offseason thus far, adding Corbin Burnes to a rotation that already boasted Zac Gallen, Merrill Kelly, and Eduardo Rodriguez. Though each of these starters has an impressive track record, a former top prospect rounds out the mix and may not be an afterthought for too much longer.
Brandon Pfaadt has yet to deliver on the potential he showed in his rise to the majors, where he debuted in 2023 and put up a subpar 5.72 ERA in 96 innings. He followed that up with a mediocre sophomore season, posting a 4.71 ERA in 181.2 innings last year. So far, only his 3.27 ERA in the surprising 2023 Diamondbacks playoff run has displayed the full breadth of his potential.
However, a look under the hood shows a bright future in the desert for the 26-year-old.
While Pfaadt’s ERA was severely below league average in 2024, the ERA estimators shown above all graded out above league average. If you prefer to see the stark difference in more recognizable terms, his 4.71 ERA looks highly suspect next to his 3.61 FIP, 3.75 xERA, 3.58 xFIP, and 3.65 SIERA. By all measures, he underperformed by about a full earned run per nine innings.
Two likely culprits of his disappointing results are his poor BABIP and LOB% showings. While there is a minor amount of skill in these stats, they are largely luck-based and typically regress back to league average given enough time. It wouldn’t be surprising to see more outs on balls in play and less bunching of baserunners in 2025, leading to better results even without improvement in overall skill.
However, Pfaadt may see a bump in his skills anyway. Stuff+ and PitchingBot are two models developed by Eno Sarris and Cameron Grove, respectively, which attempt to remove extraneous factors and measure the raw ability of a pitcher’s pitches. They each have a Stuff component and a Location/Command component, which combine to give the overall (OVR) values I’m using here.
As we can see, Stuff+ and PitchingBot agree that Pfaadt’s pitch mix is already elite. That is somewhat apparent in his above-average strikeout rate and excellent ability to avoid walks, but there appears to be more room for growth. There are certainly improvements to be made to Pfaadt’s underwhelming Baseball Savant profile, after all.
When these models don’t parallel a pitcher’s results like this, it’s often because he isn’t optimally utilizing his pitches. There may be hope on the horizon, though, with new pitching coach Brian Kaplan coming to Arizona. Kaplan was previously the Assistant Pitching Coach in Philadelphia, where the Phillies led MLB in fWAR during his time in the role. Perhaps he can help Pfaadt fully realize his talent.
With a new pitching coach at the helm and four excellent starters ahead of him in the rotation to show him the ropes, Pfaadt should continue his growth into the bona fide pitcher he seems destined to be. The NL West will be a bloodbath, and Pfaadt could be a determining factor in its outcome.
Spencer Schwellenbach
Heading into the 2024 season, the Atlanta Braves were considered by many to be World Series favorites, thanks in large part to their dominant rotation fronted by their young ace, Spencer Strider. Unfortunately, the injury bug chewed away at their dynastic hopes, taking out several key players throughout the season, including Strider after only two starts.
Luckily, they had another Spencer S. waiting to fill in the gaps. Though Spencer Schwellenbach is not quite the overpowering force that Strider is, he gave Atlanta 123.2 innings of 3.35 ERA pitching in his rookie season in 2024. Considering the Braves barely squeaked into the playoffs, his contributions over that of a replacement player (2.7 bWAR) very likely made the difference.
And trust me when I say: there’s more where that came from.
Schwellenbach was an above-average pitcher in 2024 by ERA, and all of his ERA estimators (of which only FIP is shown above) agreed with his results. While a 55 on the LASR scale is hardly eye-catching, if he is able to put up a full season similar to his 2024, he would prove to be a valuable piece of the Braves rotation.
He has already shown he can strike batters out and prevent walks. The potential for improvement, though, is why a true breakout to frontline starter status may be in the cards. Schwellenbach got batters to whiff, chase, and miss barreled contact at superb rates in 2024, and the Stuff+ and PitchBot models agreed that his pitch mix is of ace-level quality.
My LASR stats, however, are still selling him short. The baselines for these stats are established by league qualifiers, and all players who didn’t reach the qualifying threshold (162 innings for starting pitchers) have their LASR grades modified by a scalar equal to their progress toward qualification.
For Schwellenbach, who tossed just shy of 125 innings, that means all of his underlying z-scores (the number of standard deviations from the mean) were multiplied by 123.67/162, or about 0.76. Of course, I account for playing time for a reason — to avoid small sample pitfalls — but if he could’ve maintained his performance over a full 162 innings or more, his LASR scores would’ve been even more impressive.
That fact can be seen in his Baseball Savant profile, where his 96th percentile Chase%, 95th percentile walk rate, and 90th percentile Barrel% stand out among other excellent qualities. He also doesn’t appear to have had any significant weaknesses crop up so far, though going into his sophomore season, it will be interesting to see how he handles opponents’ adjustments now that they’ve seen him before.
The Braves will be crossing their fingers they have better luck with injuries in 2025. Schwellenbach missed the 2022 season due to Tommy John surgery, so he should have no lingering issues from the injury come next season. With Strider returning to a rotation that was bolstered by Chris Sale and Reynaldo Lopez in 2024, Atlanta hopes to enjoy a deep playoff run, and Schwellenbach will be a key piece to the puzzle.
Hunter Brown
The typically sure-footed Houston Astros shocked everyone in 2024 with a massive stumble out of the starting block, beginning the season with an ugly 7-19 record. Even after they worked their way out of that rut, it took a while to kick the gears into motion. They faced their largest deficit in the AL West on June 18th, when they were a full ten games behind the Seattle Mariners.
The starter on June 19th — the beginning of their destruction of that division lead — was Hunter Brown, whose season reflected that of his team’s almost perfectly. In his April 11th start in Kansas City, he faced 14 batters and 11 of them got base hits. His final line was absurdly rough: nine earned runs in just 2/3 innings pitched. It was his third start of the young season, and his ERA stood at 16.43.
Of course, as we all know, the Astros are inevitable. Despite the hole they dug themselves into, they won the AL West, and Brown enjoyed a remarkably similar turnaround.
Brown, who had excelled in a short debut in 2022 after showing promise as a prospect often compared to his idol Justin Verlander, followed up that cup of coffee with a replacement-level showing in his first full season in 2023. That disappointment, coupled with his poor start to 2024, had many wondering if he would be another bust in a long line of once-promising pitcher prospects.
After his Royal Disaster in April, though, Brown put up a sparkling 2.88 ERA in 162.1 innings, finishing the season with a respectable 3.49 ERA. Regardless of whether his rebound was due to regression or a true change in ability, there is more reason for optimism now than if the course of events had been reversed.
As can be seen in his LASR grades above, the biggest concern in his profile is his walk rate. However, his overall 2024 rate of 8.4% is somewhat skewed by his early-season performances. From May 11th on, that number was 7.3%, which isn’t great, but it’s certainly better and may indicate he figured something out that will carry over into further improvements.
Though he didn’t quite strike out batters at the commanding clip he showed in the minors (or even in his subpar 2023 season), he established himself as a master of soft contact. His Prevent EV (Exit Velocity) score of 70 put him in a tier behind only Michael King (75), and his Prevent Barrel% score of 75 was achieved by only him and Andre Pallante in 2024.
His effective groundball rate is another point in his favor and should help him immensely in Houston. Even when batters put the ball in the air, he is adept at inducing easy pop-ups (Infield Flyball – IFFB%), which are practically just as valuable as strikeouts. He also suffered from a poor BABIP showing in 2024, providing hope that a natural bounce back toward league average will further improve his results.
His relatively poor control requires caution, but if he can continue to make strides in that area, Brown has the makings of a potential ace. With the Astros’ knack for developing stellar pitchers, it wouldn’t be surprising to see Brown blossom into the next headliner of a Houston team that refuses to bow out of competition.