Quantifying why Jake Peavy’s been superb with the Giants

Jake Peavy‘s been absolutely stellar to the tune of a 2.29 ERA and 2.73 FIP since joining the San Francisco Giants in late-July. While everyone anticipated Peavy to improve pitching half his games in the spacious confines of AT&T Park, no one expected him to be this good. And this good is really good; ranking with one of the top-ten lowest ERAs in the National League the second-half of the season.

The player who’s been present with the Giants is a far-cry from the one who was with the Boston Red Sox. I mean, the guy sported an abysmal 4.72 ERA and 4.79 FIP with the cellar-dwelling Red Sox. Like I said, improvement was very much expected; however, the difference is too dramatic to bat an eye and say definitively “this is simply due to the transition from a right-handed hitter’s paradise (Fenway Park) to a hitter’s nightmare (AT&T Park).”

Now, the easy and immediate assumption would be the right-handed pitcher has been the benefactor of a lot of luck since joining the Giants back in July, but that’s just not true. His BABIP (batting average on balls in play) with Boston sat at .301, and with San Francisco’s it’s been a crisp .294, which is only .02 percentage points below league-average and is .09 percentage points higher than his career .285 BABIP. Further underplaying luck’s role in Peavy’s prosperity, his 73.2 LOB% (left on-base percentage) with the Giants is actually lower than it was with the Red Sox (74.3 LOB%).

So, yeah, it’s not a “luck” thing. Rather, it seems a change in his pitch selection has been the difference-maker for his success. According to Brooks Baseball’s Pitch F/X data, Peavy used his slider the month of August (the only full month Peavy’s pitched with the Giants) the most frequently in any single month since July 2012. Of course, going to the breaking pitch more often would’ve proven futile had he not located and sequenced the pitch well, but considering hitters slugged a paltry .091 off the pitch in August, it’s probably wise that he mixed it in more. In that same month, the 33-year-old hard-nosed pitcher also used his curveball more often than any one month since 2012. He didn’t have the same success he did with the slider in August, yet batters only slugged a meager .360 off it.

Continuing to sequence in September as he did the month before seemed logical, but Peavy has done anything but. In fact, he’s thrown the slider nearly half as much and the curve a fraction less than he did in August, and, surprisingly, it’s wielded him positive results. And when I say “positive” I’m referencing a pretty solid 0.77 ERA in two September starts while throwing his four-seam fastball over half the time.

Baseball’s a game of adjustments. For a pitcher, you need to constantly keep hitters guessing at what pitches you’re going to throw and where you’re going to throw it. Peavy has been unpredictable the past two months, and that’s made a huge difference. The fact the notorious fly-ball pitcher got out of pitching at Fenway didn’t hurt, either, which is evident by his 0.31 HR/9 ratio with the Giants. All said, a combination of more feasible working conditions and keeping hitters guessing have delivered better results for the proud duck boat owner.