The month of April is over, and there is no shortage of unusual baseball story lines to follow. One of interest is Jordan Zimmermann, the recent free agent signing of the Detroit Tigers, who leads all of baseball with a 0.55 ERA through five starts. While it is fairly evident that he or any pitcher will not end a full season with a 0.55 ERA, encouraging peripherals point to a legitimately improved start to the season.
Through 33.0 innings pitched, Zimmermann has featured a 2.69 FIP, a higher than usual 47.5 percent groundball rate, and a consistently strong pop-up tendency with a 10.8 percent infield fly rate. The batted ball results are great, but the defense independent numbers might be a little misleading. It is unusual to have FIP drop by more than a full run despite worse strikeout and walk rates. His 17.4 percent strikeout rate and 12.1 percent strikeout minus walk rate are each the lowest figures of Zimmermann's career.
The obvious explanation to these conflicting numbers is Zimmermann's abnormally low home run rate. In April, only the Twins' Byung-ho Park was able to take him deep, leading to a 2.7 percent home run per fly ball rate that is unsustainable and below his career average.
However, he's been a very effective pitcher in prior years without the benefit of a low home run rate. The lack of strikeouts he's generated is a little more concerning. His fastball velocity has dropped over a mile per hour (91.7 mph) from last season's already decreased rate (93.0 mph). Batters aren't swinging at pitches outside of the zone anymore (down to a 21.7 percent O-swing rate from 32.5 percent in 2015), and he's generating fewer swings and misses.
This information seems to paint a negative picture moving forward for the pitcher, and if he were not to adjust to these complications, that might be true. Instead, Zimmermann has made some changes, at least early on in the season. Primarily, these changes have revolved around his swing-and-miss secondary pitch, the slider.
Zimmermann's slider has always been the kind of pitch that has succeeded with pinpoint command over exaggerated movement. His 1.5 inches of horizontal movement is nothing special, as far as sliders go. However, that reliance on command over run means it might not suffer from platoon splits as much as other pitchers' sliders. Brooks Baseball reports that for his career, left-handed hitters have performed even worse against the pitch than right-handed hitters. He's just never relied on the pitch against opposite-handed batters — until this year.
The jump in total usage of the slider is entirely due to using it more often against left-handed hitters. Zimmermann's usage patterns haven't changed at all against right-handed hitters, and in April he even used the pitch slightly less than in 2015. With a career-best 57.1 percent zone rate, Zimmermann can throw the pitch to any part of the plate for a strike. In fact, Zimmermann seems to be locating the pitch differently now that it does the heavy lifting against opposite-handed hitters. Where before the slider's location against lefties was mostly indistinguishable from its location against righties, it has been targeted more distinctly on the outer half against them in 2016.
The slider's current 64.1 percent groundball rate is almost 20 percentage points higher than his career average with the pitch. That's important because his fastball has actually generated fewer groundballs than in the past. The fastball has a career 40.6 percent groundball rate but has had only a 30.4 percent rate this season. Some of that isn't bad — the pitch has seen an additional 1.9 inches in rise over his career rate, which helps generate weaker fly ball contact. However, there may be some regression against the pitch in the home run department.
Jordan Zimmermann is now on the wrong side of 30 years old; his velocity probably isn't going back to its prior levels. His future Major league success will rely on continued fastball elevation and command of his repertoire, most notably his slider. Early in the 2016 season, that command has reliably generated outs of both left-handed and right-handed hitters, and some good fortune with home runs has left him atop the ERA leaderboard.
Moving forward, Zimmermann will likely experience regression related to home runs, and I wouldn't expect him to be leading the league in ERA after 30 starts. However, learning to rely on his slider against both left- and right-handed batters may offer a solution to the platoon split that haunted him in 2015. If he can maintain his command going forward, Tigers fans will likely be happy with the team's five-year investment.
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