In mid-July, Texas Rangers' LHP Martin Perez returned from Tommy John surgery. After flailing a bit out of the gate, Perez has righted the ship in time for the playoffs - even though run prevention numbers still somewhat belie the quality of his total performance.
The most obvious and cliched indicator of this is the margin between his 4.46 ERA and 3.41 FIP, but interestingly that margin isn't buoyed by the strikeout rates one might expect from a former top prospect.
Perez has never matched his low minors strikeout numbers in the majors, with a career 15.3 percent rate. However, he's featured an above-average ground ball rate, and that has in turn helped limit home runs allowed, particularly this season.
Below are Perez's peripheral rates from Fangraphs, comparing his post-return performance in 2015 to his earlier career (2012-2014).
To summarize, while he is striking even fewer batters than before, he's generating a lot more weak contact. His hard-hit rate has dropped by a quarter, his ground ball rate has increased to the fifth-highest rate among starters (minimum 70 innings pitched), and his walk rate has dropped to roughly league average.
So far, he's gotten batters to chase more often, make weak contact, and generate ground balls. If counting four-seam fastballs and sinkers separately, Perez throws a five-pitch mix. Where is this new weak contact coming from, exactly?
The sinker and changeup have become Perez's primary arsenal, and on the year both have outpaced the four-seam fastball in terms of usage. In particular, the off-speed pitch has seen a large increase in usage - however, it is not seeing a significant increase in ground balls. It's actually generating fewer whiffs and strikeouts than in the past (data from Fangraphs).
|BB %||K %||GB%||SwStr %||BB %||K %||GB%||SwStr %|
However, Perez has managed to cut the pitch's walk rate almost in half, and for a change-up the ground ball rate is still very high, with a roughly average whiff rate. It's a good pitch - but what is really interesting is the effect it might be having on his sinker.
Skimming the table, it's obvious that the sinker, already an above-average pitch, has jumped into a new tier with regard to weak contact. It's possible that this has to do with the interplay of the two pitches, which have developed strikingly similar movement over the last few weeks.
With pitches that move similarly like that, the roughly seven-mile-per-hour difference in velocity might be enough to throw off batters' timing. In fact, Perez's largest percent usage of the changeup comes with right-handed batters ahead in the count - typically, if a strike is needed, a fastball might be used there. Indeed, the sinker also sees its largest percent usage in the same situation. The sinker-changeup interplay is a big part of Perez's game when he is behind in the count.
The two pitches are also used in almost the same locations. The changeup catches a little bit more of the bottom of the strike zone, but anywhere the sinker is used at 92-93 MPH, the changeup might be used with the same movement at 85-86.
Moving on from the changeup and sinker, the slider has also seen a jump in ground ball rate. Perez uses his slider more frequently against same-handed batters. The most notable feature of this pitch is its jump in velocity - a pitch that has sat in the past between 84-85 is averaging 86.8 MPH in 2015 (per Brooks Baseball). As a result, it has maintained most of its bat-missing abilities while the ground ball rate has jumped 11.9 percentage points (see the above table). The ground ball rate is likely a by-product of a very high 44.9 percent chase rate (per Fangraphs).
The slider gives Perez three potentially strong pitches and the ability to get out batters of either handedness. The four-seam fastball and curveball have been inconsistent offerings this season, and at their best have been used to show batters a different look (the curve has been used most frequently as a first pitch).
It would be easy to recommend scrapping these final two pitches, due to each's ugly stat line, but it is very difficult to calculate how much the more promising offerings rely on the threat of something unexpected keeping batters off-guard.
Since returning from Tommy John surgery, Martin Perez has both generated more ground balls and walked fewer batters than he has in the past. Strikeouts have lagged behind these other peripheral indicators of success, but as a young pitcher with top-prospect pedigree recovering from surgery, there may still be potential for growth in that area as well.
For the time being, Perez is still a better option in the rotation than his partial-season ERA would indicate, an option in which the Rangers can feel confident in entrusting a back-end spot in a playoff rotation.
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Spencer Bingol is a Contributor at Beyond the Box Score. You can follow him on Twitter at @SpencerBingol.