You’ll Never Guess What Spencer Schwellenbach Will Do Next

I’m not much of a YouTube guy or, really, a fan of videos in general. If you send me an Instagram reel, I’m sorry, I will not watch it. But Lance Brozdowski delivers his baseball thoughts in video form, so I am compelled to make an exception. Lance’s posts prodded me to start writing about baseball in the first place; I always learn something when I watch his stuff and tend to agree with all of his analysis.

So I was shocked — shocked! — to hear him express pessimism about Spencer Schwellenbach in a recent video. All through this offseason, I’ve had the opposite thought: There isn’t enough enthusiasm about Schwellenbach’s rookie campaign, during which he posted a 3.29 FIP over 123.2 innings. But Lance wasn’t the only one with a tepid appreciation for the right-hander. Eno Sarris ranked him as his 34th-best starting pitcher; Thomas Nestico had him at no. 36. If I were obliged to make such a list, I might be pushing him some 20 spots higher. I think Schwellenbach’s rookie excellence can be repeated and even improved upon for one key reason: When he delivers the baseball, nobody knows what to expect.

Consider Schwellenbach’s scores when it comes to Baseball Prospectus’ new arsenal metrics. These metrics analyze a given pitcher’s arsenal and grade it across four categories: Pitch Type Probability, Surprise Factor, Movement Spread, and Velocity Spread. As I wrote in January, “Each score is converted into a percentile form, allowing for easy comparisons between pitchers. The knuckleballer Matt Waldron, for example, ranks as a 98 in Surprise Factor, the highest among all pitchers.” It’s these first two qualities — Pitch Type Probability and Surprise Factor — where Schwellenbach stands out among his peers. Against hitters from both sides of the plate, Schwellenbach uses his wide mix and the overlapping movement between his pitches to cultivate confusion. As Michael Baumann wrote about Schwellenbach last September, “He can throw anything, anywhere, at any time.” Take a look at his arsenal scores:

Schwellenbach even remains unpredictable at the count level. There isn’t a single count — to righties or lefties — where Schwellenbach threw a pitch in even 50% of situations. His most predictable pitch/count combination was his slider to right-handed hitters in 1-2 counts; in 105 of these situations, he opted for that pitch 50 times, or 47.6% of the time. For comparison, Logan Gilbert — who also has six pitches and throws his four-seamer a virtually identical amount of the time — had seven different pitch type/count combinations above 50% usage. (In 1-0 counts to lefties, for example, Gilbert threw his slider nearly 60% of the time.)

Schwellenbach’s 0-0 tendencies illuminate the difficulty of sitting on one specific pitch against him. To lefties in 0-0 counts, he threw his curveball 35% of the time and his four-seamer 33% of the time; there was roughly a one-in-three chance you’d see one of his other four pitches. To righties on the first pitch of a plate appearance, the curveball dropped down to 11.4% usage; the fastball was thrown 38% of the time, the cutter 23.6%, the slider 19.2%.

But he isn’t just a crafty deep-mix guy — the stuff is nasty. Schwellenbach throws three fastballs, including a four-seamer that averaged 96.1 mph in 2024, ranking in the 90th percentile of all starters last season. The shape on that four-seamer, however, is so-so — his low arm angle and tendency to cut the baseball means it doesn’t get much of that bat-missing vertical hop:

It’s really the offspeed stuff that sets him apart. Against right-handers, Schwellenbach wields a filthy slider, thrown with plus velo and sharp two-plane break. For lefties, there are two options to induce swing-and-miss: a splitter, dropping over a foot below his average four-seam fastball, and a low-80s curve, tunneling right underneath his harder slider.

Impressively for a rookie who converted to pitching full-time just three seasons ago, he manages to throw all six of his pitches for strikes. The location plots suggest that Schwellenbach is a control-over-command type of pitcher at this stage of his career, preferring to challenge hitters over the plate rather than aim for fine targets. Consider the locations of his four-seam fastballs against right-handed hitters. Perhaps there’s a slight tendency to target the outside part of the plate, but that’s a lot of fastballs over the heart of the strike zone:

That sort of aggression can be accommodated when hitters have such a dim idea of which pitch is coming next, not to mention so little time to react to it. And it certainly kept the walks down: His 4.6% walk rate was eighth among all pitchers with at least 100 innings pitched last year.

More than any specific component of his game, Schwellenbach’s excellence can be best explained by how it all fits together. Only a handful of starting pitchers manage to space their pitches apart while simultaneously making them look similar out of the hand. Generally, it’s two types of guys. There are the Patrick Corbins and Miles Mikolases of the world, overcoming subpar velocity with deception and guile. And then there are the pitchers who manage to combine these traits with top-tier velocity.

Using a similar methodology to Baseball Prospectus, Driveline’s Mix+ and Match+ models measure the movement spread of a given pitcher’s arsenal and the similarity of the movement out of the hand, respectively. There were three pitchers who posted at least a 105 Mix+, a 105 Match+, and averaged at least 96 mph on their four-seam fastball: Paul Skenes, Tarik Skubal, and Schwellenbach. (In the interests of not picking arbitrary endpoints to make my point stronger, I’ll concede that Cole Ragans and Nathan Eovaldi missed the fastball velocity cutoff by 0.6 mph.)

Schwellenbach’s pitch plot shows how he accomplishes this feat. Unlike nearly any other pitcher in baseball, his pitches touch all four quadrants of the plot, simultaneously covering a ton of space while creating bridges between each pitch:

It’s worth considering Lance’s doubts in detail. As he points out, Schwellenbach wasn’t as good against left-handed hitters. His strikeout rate vs. lefties — 21.8% — was roughly league-average. And the flip side of Schwellenbach’s wide arsenal is that there isn’t necessarily a new trick to unlock that will solve this problem. Pitchers like George Kirby, Gilbert, and Bryce Miller ascended to new levels by adding pitches to their repertoire and subsequently cutting their heavy use of their four-seam fastball. Schwellenbach is already near the bottom of all starting pitchers in primary fastball usage; presumably, it can’t go much lower than 30%. If he’s “maxed out,” as Lance puts it, that means he might not have a trick up his sleeve against opposite-handed hitters.

But I have a sense that Schwellenbach might be able to assuage Lance’s doubts with a straightforward tweak: increasing his early-count cutter usage.

Schwellenbach preferred to throw his cutter to right-handed hitters, targeting the outside of the plate where it tunneled well with his four-seam fastball. That was on display in his final start of the season, a seven-inning gem in that wild first game of the September 30 Braves-Mets doubleheader, before everything went nuts. He threw zero cutters through the first four innings; it finished as his most-used pitch that day. He threw 24 cutters to righties, using it as his primary fastball the second and third time through the order; he threw just two to lefties.

In the regular season, the cutter was his fourth-most used pitch against left-handed hitters; he threw the four-seamer with double the frequency. Lefties did the bulk of their damage off that four-seamer. If he reached two strikes, he was in a good spot — lefties whiffed 45.8% of the time they swung at his splitter. But to deploy that chase pitch, Schwellenbach first has to get to two strikes. Last season, he mostly used the cutter when he fell behind lefties, trying to induce weak contact when his back was against the wall. (It was his most-used pitch against lefties in 2-0, 2-1, and 3-1 counts.) Schwellenbach could choose to increase the usage of the pitch in early counts, preventing opposite-handed hitters from hunting four-seamers before the arrival of the dreaded two-strike splitter.

The fastball shape is far from ideal; Schwellenbach still has some things to figure out against lefties. But I just don’t see some big correction happening. He commands a wide mix, wields weapons to both lefties and righties, throws his heater with plus velo, and even fields his position with the best in the business (his +5 DRS was 10th among all pitchers). And if a pitcher refuses to be predictable, how are hitters supposed to adjust?

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