Jordan Zimmermann's Disappearing Fastball

April is riddled with false conclusions and tiny sample sizes, but there are some numbers that can be useful this early. One of them is fastball velocity and it paints a troubling picture for Jordan Zimmermann.

This post was originally going to be about all of the Nationals pitchers average fastball velocity, with fastball velocity being one of the few stats that stabilizes in such a small sample size. It would’ve been an easy post, with a chart of all the Nats pitchers and how their velocity compares to last season. However, when I began researching for the piece I noticed something much more concerning: that Jordan Zimmermann’s fastball velocity is down by a bit over 2 miles per hour. So today we’re focusing on him.

Zimmermann has had an awful start to the season. Over three starts he has a 6.14 ERA, 4.36 FIP, 10.3 percent strikeout rate and 7.4 walk rate in 14.2 innings pitched. Normally, that’s something that would be glanced over immediately, chalked up to a small sample size fluke. But in this case it is accompanied by something even more concerning a precipitous drop in his average fastball velocity.

The last three years Zimmermann had incredibly consistent velocity, averaging 94.63, 94.67 and 94.58 miles per hour. After 161 fastballs thrown this year Zimmermann is averaging 92.56 miles per hour, a difference of 2.02 miles per hour. That puts him tied for 15th among starting pitchers in largest drop in average fastball velocity.

His max fastball velocity doesn’t paint any prettier a picture. Using Baseball Savant’s PITCHF/x tool, I found that Zimmermann has only thrown two four seam fastballs above 94 miles per hour. A 95.2 mile per hour ball to Odubel Herrera and a 94.2 mile per hour pitch that led to a Freddy Galvis lineout. In other words, through three games Zimmermann has only thrown one pitch faster than his average velocity the previous three years.

In the seminal public piece on the effect of fastball velocity on pitcher performance, Mike Fast at The Hardball Times found that starting pitchers can see their overall run average increase by about .28 runs per one mile per hour decrease in fastball velocity. Last season Zimmermann had a run average of 3.02, so an increase of .56 runs wouldn’t suddenly make him a bad pitcher. For context, Stephen Strasburg had a run average of 3.6 last season. But that’s just an average, not a hard and fast rule that each pitcher follows and such a sudden change could lead to a larger decline in performance.

Not to mention that there have been some other troubling signs too. Not only is Zimmermann throwing slower, he isn’t tricking opposing batters. According to FanGraphs, opposing hitters have a swinging strike rate of 4.2 percent, less than half of his career average of 8.6 percent. And by PITCHF/x’s plate discipline stats, opposing batters are swinging less frequently at pitches outside the zone (22.6 percent compared to 32.4 percent for his career) and making more contact (91.0 percent compared to 82.4 percent for his career). Swinging less and making more contact is the opposite of Reese’s Peanut Butter Cups when it comes to desirable pairings.

I’m not going to speculate on the reasons why or what this means for Zimmermann in the long term, but these aren’t numbers you want to see from one of the lynchpins of the Nats’ team-carrying rotation. Not all of his problems are because of velocity, but it’s the only one he likely can’t fix easily. As the season continues Zimmermann will have to adjust to the decreased velocity to continue to be the star pitcher he and the Nationals expect.

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