Mariners LHP Justus Sheffield’s success depends on the fastball

If we do get to watch Seattle Mariners baseball this year, very few players will be more interesting to track than left-handed pitcher Justus Sheffield. But his success isn’t about his off-speed stuff.

The Mariners would like to find out whether or not Justus Sheffield is a member of their next playoff rotation, a high-leverage reliever, a long reliever, or trade bait to acquire somebody who will help the next playoff team. 2020 was going to be a huge step in that process, but that has obviously been pushed back.

But whenever Sheffield does take the mound for Seattle, his success will almost certainly be dictated by one pitch. Most of the talk last year was about whether or not Sheffield could find a 3rd offering to allow him to get through the lineup 3 times. We spent a lot of time talking about Sheffield’s changeup, but the truth is, that isn’t the pitch that needs improving.

In order for Sheffield to truly reach his ceiling, he needs to maximize the value from his fastball. Velocity isn’t the issue, Sheffield sits 92-93. But the four-seam fastball has the lowest spin rate in the majors and Sheffield doesn’t locate it consistently enough.

This is evident from the starts he made in 2019 and backed up by the numbers. Against Sheffield’s fastball, opponents hit .299 with a .507 slugging percentage. But opponents actually had a high batting average against both of his off-speed pitches, so what gives? Well, there are two factors at play here.

First, there is a bit of bad luck present. The xBA (expected batting average) on both his changeup and slider are between 80-100 points lower than the actual average and the XSLG is between 100-130 points below the actual slugging percentage.

But second and perhaps more importantly, both the changeup and slider generate a lot of swings and misses. Sheffield’s slider generated a 47.5% whiff rate, while the changeup was at 34%. That means that almost half the time a hitter swung at Sheffield’s slider, they missed and a little over 1/3 of the time they swung at the changeup, they missed.

But what about the fastball? It generated a whiff rate of just 12.8%. This, combined with the terrible spin rates and poor location, is a recipe for a lot of hard-hit baseballs, or damage.

Thankfully, Sheffield is already beginning to make some adjustments. We heard reports of Sheffield tinkering with his fastball grip, moving from a four-seam fastball to a two-seam grip, which may generate more sink and allow for better command.

Sheffield may also be a good candidate to throw a cutter. The slider is already plus and adding a tighter version of the pitch would give him another weapon. If Sheffield cannot get value out of heater, he will almost certainly be destined for the bullpen, albeit a high-leverage one.

But Sheffield is still young and athletic and has shown to be a “worker” in his short time with the Mariners organization. Whenever Sheffield does take the mound, it will be interesting to track the growth of his fastball. But until that day, Sheffield is still a fascinating case to consider as we move forward with the rebuild.