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Justus Sheffield Is Leaning Into It

Justus Sheffield had a sinking fastball, and now he has a sinking sinker

MLB: Chicago White Sox at Seattle Mariners Anne-Marie Sorvin-USA TODAY Sports

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It wasn’t all that long ago that Justus Sheffield was a highly touted prospect. After the 2018 season, Baseball America ranked Sheffield as the 27th prospect in baseball, and he was, of course, the headliner in the James Paxton trade. Since the trade, I’ve become increasingly pessimistic that Sheffield will end up being a good starter. And for good reason! He struggled in Triple-A, got shuttled down to Double-A, and then put up shaky numbers during his major league cup of coffee. More recently, though, I’ve been coming around on Sheffield.

For the most part, I’ve been down on Sheffield for the past year because he has a bad fastball. He has some other flaws — he has historically had issues commanding it — and then the pitch itself isn’t good. That’s the main thing. We’re limited to a 321 pitch sample in the major leagues in 2019, but his fastball returned an anemic 4.4% swinging-strike percentage. Given his average 92.8 mph velocity and 1835 rpm spin rate, there’s no reason to think that it should be better, either.

A chart from Jeff Zimmerman, detailing swinging-strike percentage based on fastball spin and velocity:

There’s a lot of information to take in here, but using Sheffield’s fastball spin rate and velocity, the chart shows that fastballs like Sheffield’s put up a 4.1% swinging-strike percentage. That’s because, despite workable fastball velocity, Sheffield’s fastball ranks in the 0th(!) percentile in spin rate, and in the fifth percentile in active spin rate (64.1%). That means that not only does his fastball not spin well, but also, just 64.1% percentage of his fastball spin contributes to movement. The result is a bowling ball fastball with sink; it acts more like a sinker than a four-seam fastball.

I should note that it’s not all bad. Perhaps the most encouraging thing about Sheffield is his ground ball percentage (52.3%), which is the 11th highest number in the league of starting pitchers with 30 or more innings. That’s not an accident. That’s because of his fastball, as well as his ground ball-inducing changeup. So on one hand, his pitches allow him the ability to keep the ball on the ground. But on the other hand, the very nature of sinking fastballs is that they are put into play more often. That means two things: (a) fewer strikeouts, and (b) reliance on factors outside of one’s control (e.g., fielding, park, weather).

So what did Sheffield do? He made the most pragmatic tweak he could have. He’s leaning into it.

Theoretically, Sheffield could have tried to work on the spin efficiency of his fastball, but the gap is just too wide to close. Instead, he shifted his fastball axis even further with a new grip, and so we should see more sink and run on his fastball once we have access to his numbers.

Here he is last Sunday, throwing an 0-0 sinker to Yolmer Sanchez:

Taken for a strike, Sheffield comes back with another sinker:

In an 0-2 count, that sets up Sheffield to go for the kill. He judiciously moves away from his sinker and comes back with a secondary offering:

Sanchez is by no means a world beater. In fact, he’s a pretty poor hitter. But he’s ever so slightly above-average by strikeout percentage, swinging-strike percentage, and contact percentage, so this isn’t nothing either. In any case, Sheffield dispatches Sanchez in just three pitches, and it was the sinker that set it all up.

Again, it’s important to note the subtle changes in approach. Throughout the game, Murphy consistently sets up on his sinker down in the zone on Sheffield’s glove-side, while occasionally setting up to his arm-side. This isn’t necessarily a huge departure from what Sheffield did last year, but in general, Sheffield’s four-seamers in 2019 generally ended up around in the middle of the zone. On Sunday, he peppered the bottom corners of the zone throughout his outing with sinkers, which is precisely what we want to see him do, and what he should have been doing all along. Perhaps this all has to do with feel for his new sinker.

If you ask me, this is a good change. Right now, Sheffield is in fastball purgatory, in that his fastball sinks more than a typical four-seamer, but doesn’t get the arm-side movement or sink of a sinker. Really, there’s no reason not to do this if his fastball isn’t going to garner more whiffs. He didn’t strike anyone out with his sinker (although he did so with both his changeup and slider) but he did use it to set everything else up.

It comes with an added bonus, too. Sheffield throws his changeup with a two-seam grip, and so it stands to reason that his changeup would benefit from a stronger pitch tunnel with his sinker. He’s been in search of a changeup for about five or six years — he’s said so much himself — and it seems like he’s found one. He really leaned on it in his last few games in 2019. Given that it’s a pitch with a lot of drop, I think switching out the fastball for a sinker will prove to be symbiotic for both his sinker and changeup, and if Sunday was any indication, it sounds like his new sinker will be his main offering. That’s great news, especially considering that he plans to keep the four-seam around to throw at the top of the zone.

Here’s where he’s generally thrown his fastball over his career:

Lots of plate! Too much plate!

Here’s how his fastball has induced whiffs over his career:

It’s not amazing, and the sample is small, but it’s clear that if his fastball is going to play anywhere, it’ll be at the top of the zone rather than the bottom, even despite its lousy spin rate. With that in mind, Sheffield has transformed his approach. Instead of middle-middle fastballs, we should see him use sinkers at the bottom of the zone, four-seamers up in the zone, and then use his changeup and slider to draw whiffs. Before, he had three kinds of offerings, but now he has a collection of four pitches that he’s going to use to do different things, in different areas of the zone.

Now that Sheffield has a sinker, I think Patrick Corbin vaguely resembles the formula that Sheffield should be after. The bulk of what we’re looking at is Corbin has one of the best sliders in baseball, and he throws it a lot. In terms of starters, Sheffield ranks eighth in the league in slider swinging-strike percentage, and he throws it as much as Corbin. And now, he’s got a sinker and four-seamer, just like Corbin.

We can compare them by repertoire:

Sheffield vs. Corbin, pitch mix

47.80% --- 35.70% 16.30% ---
19.40% 34.30% 37.10% 5.70% 3.50%
(Sheffield, top. Corbin, bottom.) Source: Baseball Savant

Now, I wouldn’t necessarily call this a comp, per se, as there are enough dissimilarities that it’s not super clean. But Sheffield is a unique pitcher. What I’m mostly after here is Corbin’s repertoire, as I think Corbin serves as a precedent for what’s to come for Sheffield. Whenever Sheffield finally pitches in the regular season, I think there’s a strong possibility that Sheffield’s repertoire looks more like Corbin’s in 2019 than his own in 2019, just with more changeups and no curveballs. It’s a shame that Bundy hasn’t implemented his changes yet (i.e., switch out the four-seamer for a sinker and throw more secondaries), because I think he is perhaps the strongest comp for Sheffield. That might sound bad — he’s been pretty mediocre these past four years — but I’m quite fond of Bundy.

The command woes aren’t completely gone. He still missed his spots more than a few times — sometimes by inches, sometimes by feet (which is okay, all pitchers do), but he certainly seemed more locked in than he often did in 2019. It’s too early to make too many assumptions at this point, but it’s safe to say that Sheffield is moving in the right direction.

Little by little, Sheffield is addressing his blemishes. He already started leaning on his secondary pitches more toward the end of last year, and now he’s made the obvious tweak of switching out his sinking fastball for a sinker. I’m not convinced this solves all of his problems, but it solves a problem — and a significant one, at that. Sheffield was misusing his last fastball, but now he has a new fastball that he’s using well. At least so far. This opens up the opportunity for a significant domino effect to take place. There’s a strong chance that, in leaning into the movement of his fastball, Sheffield has just accelerated his development.