These three MLB starting pitchers are destined to break out in 2025

A statistical deep dive into three future aces who are poised to turn in big seasons in 2025.

Sep 19, 2024; Milwaukee, Wisconsin, USA;  Arizona Diamondbacks pitcher Brandon Pfaadt (32) throws a pitch during the first inning against the Milwaukee Brewers at American Family Field.
Sep 19, 2024; Milwaukee, Wisconsin, USA; Arizona Diamondbacks pitcher Brandon Pfaadt (32) throws a pitch during the first inning against the Milwaukee Brewers at American Family Field. | Jeff Hanisch-Imagn Images
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Spencer Schwellenbach

Spencer Schwellenbach winds up to throw a pitch
Sep 30, 2024; Atlanta, Georgia, USA; Atlanta Braves starting pitcher Spencer Schwellenbach (56) throws against the New York Mets in the second inning at Truist Park. | Brett Davis-Imagn Images

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.