Baseball's Most Underrated Player

Dissecting how a player with just 87 MLB innings can already be so good, and the adaptations he's already made.

Introduction

A recent tweet from baseball YouTuber GiraffeNeckMarc asked his followers who the most underrated player in the league was. I took a minute to think about it, as many tweets from Mr. Luino tend to do, and then realized the answer was quite obvious. It’s Spencer Schwellenbach, and it’s an opinion that I’ve held, without consciously realizing it, for a while. In fact, my appreciation for him started after just three or four starts, when he had a 4.98 ERA with several clunkers in his short career. At that point, I bought him in TGFBI, a free-to-play fantasy baseball league for notable fantasy baseball analysts and players, for the highest price I’ve paid all year, $123 (out of a $1000 budget). I was willing to pay so much, despite his results to that point, because I believed that his arsenal showed potential far beyond what he had demonstrated in his first few weeks. I believe he has begun to live up to that potential, and I hope to convince you to hold him in the same regard as well.

Fastball

His fastball is probably his weakest pitch, which is funny because it’s certainly no slouch. It’s his only pertinent pitch to not have an 80th percentile chase rate, but he can pound the zone with it for some great results at 96 mph with 74th percentile extension and above-average vertical approach angle (VAA). The results themselves have been polarized: he runs a 20% called strike rate against left-handed batters (LHB) without getting crushed, while against right-handed batters (RHB), he has an absurd combination of metrics: 39% CSW, 63% groundball rate, 37% ideal contact rate (ICR), and 34% whiff rate on 29% usage. I think the kitchen-sink approach helps a lot here, and he also has played with the pitch to make the movement more unique relative to his arm slot. The man himself talks about it in this Eno Sarris tweet. This adjustment has made him more confident in the pitch, and he’s correspondingly used it more in place of his sinker in the last month or two. Former Astro analyst Max Bay has made a graphical representation of this added deception, which I’ve included below.

A representation of Schwellenbach’s deception from his arm angle, created by former Astro analyst Max Bay. The background colored circles represent the “expected movement” based on his arm angle, and the range of the observed movement is the red circle. One can say it is quite deceptive because the vast majority of the circle is outside the central yellow area, with the red midpoint in the second “ring.”

Slider

The 87 mph slider leads the way against right-handed hitters, at 32% usage, and for good reason. It runs a 99th-percentile chase rate against them, and while it only induces an average amount of whiffs, it creates ground balls and weak contact more than two-thirds of the time. It’s used more sparingly against lefties, but produces some of the whackiest outcomes I’ve seen in some time. It has a 53% chase rate and 21% swinging strike rate, but also gets hammered when there’s bat on ball. He currently lands sliders in the lower third of the plate 86% of the time, and when that regresses to the mean, I expect the whiffs to regress with it.

The pitch’s great location on the pitch may be more sustainable than I may give it credit for, though. Lance Brozdowski highlighted its improved location against RHB with the graphic below on Twitter, which coincides with a large increase in its performance across the board. Overall, the pitch has contributed a 29:2 K:BB ratio so far, and he seems more committed than ever to avoiding leaking it over the plate at all costs, in the same vein as Blake Snell. In his last seven starts, his zone rate against RHB has been well below-average at 36% against RHB. 

Schwellenbach is already making adjustments to his approach to make himself even better. The “last 7 starts” cover every start since his July 21 start vs the Cardinals.

Cutter

Another polarized pitch here with an identical 18% usage against both sides at 92 mph. With lefties, it’ brings a 48% chase rate and 34% ICR with an above-average amount of groundballs, while against righties, it brings a whiff rate 13% better than average and 37% CSW that also gets hit hard for a .530 xWOBA. He pounds the bottom glove-side corner against both sides, but righties seem to be able to get to it frequently. This surprises me to some extent; it tunnels very well with his slider and fastball, and they’re generally able to escape punishment. I would say that this pitch has the most room to grow out of the six, but it’s already doing a fine job. 

Curveball

Spencer’s curve may be his best pitch, and that’s saying a lot with the slider I’ve discussed and the splitter that I’ve yet to. It’s a sweeping curve with 10 inches of horizontal break, and comes in a tick harder than average at 80. It demands more respect than it’s gotten vs left-handers so far, with a 41% CSW and 37% ICR on 19% usage, but it is probably even better against RHB, with an absurd 50% chase rate, 26% swinging strike rate, 38% CSW, and 35% ICR on 10% usage. Like the slider, he never leaks it up, yet maintains a 68% strike rate on it overall. Spoiled Spencer really gets to have it all.

Splitter

Guess what. The splitter is (arguably) his third-best pitch, and has a 25% swinging strike rate, 41% chase rate, 46% whiff rate, and 30% ICR with immaculate arm-side location down in the zone against LHB. It also has a 15-inch differential in horizontal break with his slider, which creates a strong symbiotic relationship between the two for fooling hitters. Patently. Absurd.

Sinker

The sinker is only used 8% of the time, and isn’t absolutely ridiculous. Thank the heavens. It seems to be more of a pitch to add flavor than anything, and as mentioned before, has seen his role diminished as the fastball has taken more and more stage time. I’d argue it’s his worst commanded pitch, and while it can steal strikes like one would hope, the copious amount of groundballs it provides are usually hit hard enough to find outfield grass. That being said, most pitchers would love this good of a pitch to be used just 8% of the time. Frankly, part of the reason it isn’t better is because it doesn’t need to be.

Schwellenbach’s overall breakdown of his arsenal. The template is courtesy of TJStats on Twitter.

Moving Forward

Schwellenbach has everything going for him. A 6-pitch mix where 5 pitches are above-average and three are well above-average, with each bringing between 8% and 26% usage overall, on 5 different, well-spaced speed ranges. He gets tons of chases, whiffs, called strikes, ground balls, and weak contact, while working efficiently at 5.8 IP/start. In the last two months, he’s gone more than 6 per start.

I’ll now take the opportunity to note some fun facts about Spencer. Did you know that the only pitchers who have a better CSW (an indicator of fooling hitters) and ICR (an indicator of inducing weak contact) are Tarik Skubal and Blake Snell? Typically, you can only prioritize one or the other, but Schwellenbach is great at both. In a stat I made up for this article, ICR-CSW%, Schwellenbach ranks only behind Skubal and Woo amongst SP with at least as many innings. Snell barely doesn’t qualify, and obviously ranks above Spencer as well.

Also, did you know that in the last two months, he has a 70:9 K:BB ratio across 55 innings with a 33% CSW and 33% ICR? That sample includes the Phillies (x2), Coors, the Twins, and the Padres. Across a similar strength of schedule in the last two months, Skubal has a 33% CSW, 35% ICR, and an 81:14 ratio. In the last two months, Schwellenbach has arguably outpitched guaranteed AL Cy Young winner Tarik Skubal. I’ll conveniently omit Snell’s even more absurd last two months to make Spence look better.

Now, Schwellenbach is definitely not perfect. He struggles some with LHB because he indexes heaters and sliders too much and they get hit hard as a result. But the answer is quite apparent: turn to the 48% chase rate cutter, 41% CSW curve, and 46% whiff rate splitter more than a collective 58% of the time. He’s found himself hesitant at times to turn to the cutter in those situations specifically, and it’s causing his outstanding overall process to not line up with the box score. Some of that is the result of a small sample - Schwellenbach has just 87 major-league innings - but that also works both ways. The fact he’s this good at 24, with only 100 innings above High-A, with correspondingly little prospect hype, is unbelievable. His 107 Pitching+ ranks in the top 15 of all MLB players, amongst the likes of Glasnow and Cole. He’s no fluke any way you look at it.

For the future? I believe he's a top-15 SP right now, maybe even higher, with top-5 upside. He’s a bit like Kirby, with the zone-pounding, kitchen-sink approach, but Double-S seems to have quite a bit more upside with his far superior curve and splitter. I will have a very hard time not putting him in my Top 50 next year if this holds up, and unless someone reads this article, I may be the only one to have him even in a top 100. Regardless of what others think, he’s the real deal, and the most underrated player in baseball.