Justus Sheffield’s fastball isn’t raising his ceiling

With a huge influx of young talent into the Mariners organization this offseason, the top of their prospect rankings have been completely rewritten. The name on top of almost every updated prospect list is Justus Sheffield, the headlining acquisition in the James Paxton trade. His proximity to the majors and his relative polish as a 22-year-old gives him a leg up on the seemingly unlimited tools of Jarred Kelenic.

If you’ve read John or Kate’s evaluations of Sheffield, you’ll know that scouts love his raw stuff. He possesses three pitches that could grade out as plus once he hits his prime. His slider is clearly his most lethal weapon and his changeup shows great potential even if it’s not as polished as the rest of his repertoire. But the pitch most scouts love and give the highest grade is his is mid-90s sinking fastball. That kind of velocity from the left-side of the rubber is great, but it’s that second adjective describing his fastball that gives me pause.

Thanks to the brief cup of coffee he received at the end of last year, we have some actual pitch data to analyze. And even though he threw less than three innings at the major league level—totaling just 57 pitches—the stabilization point for most pitch characteristics is incredibly low. With that in mind, here are the relevant pitch characteristics of Sheffield’s fastball:

Despite the high velocity, Sheffield’s fastball features an extremely low spin rate. Among all pitchers who threw at least 25 fastballs in 2018, the spin rate he generated is the 14^th lowest mark (740 total sample size, including four-seam fastballs, sinkers, and cutters combined). That definitely explains why he’s able to get so much downward movement on the pitch. I would be content labeling his fastball a sinker and moving on in the analysis, but look at his horizontal movement, or lack thereof. The average sinker moves around 8.2 inches horizontally relative to a spinless pitch. Sheffield’s fastball just doesn’t feature the arm-side run that’s characteristic of most sinkers. It’s really a low-spin, sinking four-seam fastball, making it a rather unique pitch.

On the surface, a sinking fastball isn’t necessarily a cause for concern. Plenty of pitchers have built successful careers around a repertoire centered on a strong sinker. But their success is largely defined by their ability induce ground ball contact, not on their ability to generate impressive strikeout rates. Ground balls are more favorable for pitchers since they so rarely go for extra-base hits, but batters just don’t miss sinking fastballs all that often.

Generally, you want a four-seam fastball to “rise” to induce swings and misses at the top of the strike zone. A four-seam fastball that doesn’t rise usually means higher contact rates, even if that contact is put on the ground more often. Simply based on his fastball velocity and spin rate, we can estimate that Sheffield’s whiff rate would sit somewhere around 12%, dead average for a sinker but well below average for a four-seam fastball.

Based on his fastball’s pitch characteristics, I pulled a list of Sheffield’s left-handed peers to compare against. These comps possess four-seam fastballs that move like Sheffield’s, even if their velocity or spin rate might differ greatly.

As with any list built like this, the results are pretty mixed. Seeing names like Chris Sale and David Price above is obviously encouraging but that shouldn’t necessarily color our evaluation of him. You’ll notice that almost every single pitcher listed above has a spin rate at least 200 RPMs above Sheffield. Perhaps the closest comp if we take spin rate into account is old friend Mike Montgomery, but even then, Sheffield’s velocity is much higher than MiMo. It’s certainly possible that extra velocity will help Sheffield generate a whiff rate closer to Eduardo Rodríguez’s or Price’s, but that seems like a stretch given what we know about his extremely low spin rate. And for every Price or Sale on this list, there’s a Freeland or Richard to remind us of the low whiff rates a sinking fastball is supposed to generate.

For Justus Sheffield, a sinking fastball doesn’t necessarily portend doom. By all accounts, his slider is good enough to generate healthy whiff rates and his changeup is coming along too. But the lack of potential whiffs via his fastball severely limits his ceiling. A mid-rotation ceiling is valuable for the Mariners but they’ll need to find some top-of-the-rotation talent if they’re serious about contending in a few years. Paxton had that kind of talent. The return package from the Yankees was viewed as a little disappointing specifically because the Mariners didn’t receive that kind of potential ceiling in return. Sheffield will probably become a valuable member of the Mariners starting rotation in the years to come, he’s just not the ace they’re looking for.