The diverging seasons of Justus Sheffield and Justin Dunn

When the Mariners announced they’d be using a six-man rotation for the entire 2020 season, it was great news for two 24 year olds: Justus Sheffield and Justin Dunn. Each player locked in a spot in Seattle’s rotation, matching up back to back in an opportunity to get around 10 starts and continue their development while staking a claim on a 2021 rotation slot. The two are disparate pitchers, with the lefty Sheffield leaning into his sinker for a groundball profile, while Dunn uses his running four-seam to miss bats and induce fly outs. Nonetheless, they entered 2020 with fairly similar developmental goals.

Both young pitchers leaned primarily on their fastballs and breaking balls in their ascension to the bigs, but finished the 2019 season needing more from their change-ups. Relatedly, they were each subject to significant platoon splits in the upper minors and their short big league careers. Command issues also led to higher walk rates and shorter outings at times for both pitchers. Some mechanical tweaks could clearly help with consistency, but improvements on any of these fronts would help the pair of Top-100 prospects stick as starters despite concerns one or both would be moved to the bullpen. Three quarters of the way through the season, and the two are trending in opposite directions.

Sheffield’s 2020 couldn’t reasonably be going much better. The lil’ lefty has upped his rate of pitches in the zone (per Baseball Savant) from 45.6% to 50.1%, as far above league average now as he was below in 2019. The uptick in balls in the zone has cut his rate of whiffs, as has leaning fully into a two-seamer/sinker and almost entirely discarding his four-seamer, but it’s been a net positive for his results and peripherals. That’s because Sheffield, at least thus far, is being punished less as a reward for his good pitching behavior. He’s four ticks above the league average for first pitch strikes at 64.6%, and hitters are swinging less frequently when he comes in the zone. Importantly, that’s included a ~10% jump in the zone with his changeup, which has remained a sharp, diving offering even with just 5-6 mph of separation. While more contact can be treacherous, contact without extra runners on base is a lot less deadly. Moreover, the changeup has helped Sheffield manage righties. Working in the zone more has meant fewer whiffs across the board, but a sufficient slowball to give opponents three reasonable pitches to have to track.

The uptick in working in the zone has also, logically, led to a huge drop in pitches per plate appearance, dropping from a slightly above-average 4.01 in 2019 to a well below-average 3.57. The clip below sums up just about everything that’s gone well in Sheff’s kitchen this year. Despite a 1-0 count and a runner on second, he’s at just 95 pitches with two outs in the seventh. He’s rewarded for a perfectly located change with a weak groundout to cap his second straight seven inning outing.

By getting himself into good counts and upping the rate at which all three of his pitches are plausible strikes, Sheffield has gotten the cumulative benefit of his many incremental improvements. The next step is likely finding a way to incorporate a four-seam at the top of the zone, however the improvements in command and adjustments after his couple poor outings are immensely encouraging. This version of Justus Sheffield is a starter, and a solid one, far more than the 4-6 innings of high effort Ks and BBs that he’d been trending towards at times over the past few years.

Then there’s Justin Dunn. Oh dear.

I’ve come this far without mentioning Dunn has a 4.11 ERA to Sheffield’s 4.06, and I won’t dwell on it much more, but it has been simply astounding to see Dunn wriggle out of jams of his own creation all year. His fastball has dodged barrels, as has his still-solid slider, and that may be the end of the clear commendations. Like Sheffield, Dunn’s average velocity is a tick or so down from last year, but he has not seen the commensurate bump in command. He’s the beneficiary of a .180 BABIP thus far, as well as a healthy heaping of cluster luck, as four of the seven homers he’s given up have been solo shots, while none have come with bases loaded. And, where Sheffield can point to improved outing length as well as consistency, Dunn has just two quality starts, and just one longer than two innings with only a single walk. His issues are multifaceted, but Dunn’s struggles receive a Carnegie Hall spotlight in his platoon splits.

Last year, between AA and his brief big league cameo, Dunn was a monster against RHHs. At home in pitcher-friendly Dickey-Stephens Park or on the more balanced road, they were powerless against him and his multiple breaking balls, along with a 91-95 mph heater. Lefties were a different animal entirely, threatening him in part with greater slugging, but just as significantly due to a massive walk rate.

Baseball Reference

Those same issues have manifested early in his big league career. Dunn has an underwhelming but passable 18.1%/9.5% K%/BB% against righties in his big league career. His strikeouts climb a couple ticks to 20.2%, but his walks skyrocket to 28.1%. It’s been a horror show for him. Dunn has had poor command of all his pitches against lefties, but his sharp slider has gotten him out of trouble more than a few times. His changeup, on the other hand, has been little short of an anchor tied to his leg.

Let’s jump back to Sheffield for a moment to elucidate that further. Sheffield’s fastball has averaged 91.8 mph this year, while his changeup has been just 86.2 mph. A 5-6 mph break, as I mentioned, isn’t great, but it’s made up for by excellent drop relative to its speed, and a bit of extra arm-side run for kicks. Baseball Savant has a tool that helps us visualize pitch movement relative to league-average movement for pitches of that type+velocity. As you can see below on the right, the darker circles with the dot in the center represent Sheffield’s pitches while the lighter circles with the frills on the edges represent league-average movement for a pitch of that type within the same velocity range.

Baseball Savant

Not bad! Sheffield’s green changeup is fast, yes, but it drops almost three inches more than the average pitch a hitter sees of that type, making it all the more difficult for batters to adjust to. Logically, Sheff has gotten plenty of grounders and whiffs on the pitch. You can see, similarly, his sinker and slider have a little more bite than the average as well. Plenty of pitchers can succeed or overachieve with average pitches, but Sheffield has stuff that moves better than most, and now he’s commanding it.

Now let’s look at Dunn.

Baseball Savant

I’ve highlighted what I believe to be “The Problem”, but really there are two things that jump off the page here. First, in green changeup land, Dunn has less separation than Sheffield, WAY less movement than him or the rest of the league, and in fact is closer to throwing a mediocre, slow sinker than he is a changeup. Its lack of movement makes it non-threatening to lefties as a swing-and-miss option, his shaky command means it’s rarely been in a good position to miss barrels, and its lack of velo gap lessens its ability to get some whiffs or poorer contact purely on hitters seeing fastball out of his hand. Even when it is well located, it’s an easy pitch for hitters to track.

The second issue is his most-used secondary, the curveball. It’s mostly a slower, more sweeping version of his slider at the moment, and one that gets hit harder. Dunn doesn’t have elite spin rates, but throwing two versions of the same pitch when one works (the slider) and one is getting hit (the curve) may need to spur some changes. Whether that means altering his pitch design to try and get more of a 12-6, vertical drop or simply sticking with his slider is unclear. The curve is a pitch Dunn can get over for strikes when he’s losing his fastball command, but the cost has been a ton of hard contact. If Dunn is two pitch-changes away from being a competitive starter, where does that leave him?

We’re not likely to see a total overhaul for Dunn in his final 1-2 outings this year, but there’s a lot to worry about headed into 2021. The drop in velocity and command is a non-starter, in that he is a non-starter if he cannot get through his outings more efficiently. To do that, he’ll need to improve at least one of his pitches, or prepare to tighten up his fastball slider in the bullpen. This year unfortunately went from a chance to test mettle to almost make-or-break for both Sheffield and Dunn. Sheffield has made his case, Dunn needs a lot of work.