Arizona Diamondbacks' starter Rubby De La Rosa has emerged as a durable pitcher in 2015. After being traded from the Red Sox this past offseason, the right-handed pitcher leads the team in innings pitched and starts while ostensibly fronting one of the weaker (although young) rotations in the majors.
His results have left a bit to be desired - a 4.59 ERA reinforced with a 4.67 FIP. De La Rosa suffers from severe platoon splits (.257 wOBA against RHH, .405 wOBA against LHH). Teams have exploited this weakness by stacking lineups with lefties, and he has actually faced more of them than their right-handed counterparts.
His peripherals are also all over the place - despite having one of the higher average fastball velocities among starters (95.5 MPH), he ranks in the bottom half of qualifying starters in strikeout percentage and allows a decent, if unspectacular, number of walks. In a more weird twist, De La Rosa actually has a really strong 51.3 percent ground ball rate but also has what is currently the highest home runs per fly ball rate in baseball. This results in the third largest xFIP - FIP margin among qualified starters.
Rubby De La Rosa works with downward movement on all four of his pitches - his four-seam fastball, slider, changeup, and a sinker added before the 2014 season. This likely explains the high ground ball rate, and each of the four pitches features a rate higher than 50 percent.
Looking at these pitches with data from Fangraphs, it becomes readily apparent where De La Rosa's problems arise - his changeup and the sinker.
The four seam and particularly the slider have been strong pitches for De La Rosa this season, generating at least 50 percent ground balls and the two highest strikeout rates of his repertoire.
Despite a strong whiff rate, the changeup has always been the weakest pitch, with a career 171 wRC+ against. It seems evident that, in an effort to develop a reliable third pitch to get out left-handed batters, the sinker was added to decrease reliance on the change after the 2013 season.
Unfortunately, the sinker hasn't made matters much better since its introduction. Despite having the best ground ball rate of his offerings, batters are both swinging at, and making more contact with, that pitch in the zone than the others. It generates the fewest whiffs of his pitches, the fewest strikeouts, a lot of walks, and has an astoundingly high 72.7 percent HR/FB rate (small sample warning, the pitch has resulted in relatively few fly balls).
One problem with De La Rosa's sinker may be that it isn't offering a lot different from his existing pitches - it's movement almost exactly mirrors the changeup, at the speed of a slightly slower four-seam fastball.
In fact, while the movement isn't so exact between the two varieties of fastball, the pitches do move in similar ways with less than a mile per hour separating their average velocities. It is possible that hitters are better able to time both the sinker and the changeup. The first appears to be a slightly lower fastball, while the latter's velocity difference compared to the four-seam fastball appears smaller than reality due to the sinker existing with similar movement, creating a wider velocity range among hard pitches.
De La Rosa's sinker hasn't been a universal hindrance this season, though. The below table from Brooks Baseball indicates that while right-handed hitters have yet to be effective against the pitch in a still-small sample, left-handed hitters have predictably crushed the pitch.
It may be an interesting idea to experiment with not throwing the sinker to lefties, who were likely the intended target of the pitch when it was developed. Instead, there has been actual improvement with the changeup that might be obscured by the sinker and may improve results.
The pitch's velocity margin with the four-seam fastball is actually the widest it has ever been, and he continues to generate whiffs and ground balls with the pitch. The walk rate on the pitch has also dropped dramatically year-over-year (from 13.8 percent in 2014 to 2.9 percent in 2015).
In addition, De La Rosa is locating the pitch more effectively in 2015. Compared to the prior pitch that was often left up on batters, he is now seeing less of the strike zone and is more concentrated around the bottom, arm-side corner of the plate.
However, the sinker has exhibited location issues similar to those faced by the changeup of past seasons. The pitch is left up too often and is being located middle and arm-side. When the pitch doesn't break enough, it doesn't have the benefit of at least being down on the batter, and the slugging percentage zone profile reflects this.
By removing the distracting sinker from De La Rosa's arsenal against left-handed hitters, it removes the pitch that generates the fewest strikeouts while only marginally affecting his ground ball rate. It also allows the improved changeup greater differentiation from the four-seam fastball. While it may never be an above-average offering, combining that with his two already above-average pitches could significantly improve his stock moving forward.
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Spencer Bingol is a contributor at Beyond the Box Score. You can follow him on Twitter at @SpencerBingol.