It's been nearly a year since Andy Pettitte unretired. As shocking as his return to baseball was, no one expected him to pitch like did. Through 12 starts in 2012, Pettitte finished the season with a 22.8 K%, a 6.9 BB%, and a 3.48 FIP. Though his fluke injury limited him to only 75.1 innings, he held a 2.87 ERA, and continued to post numbers in the range of CC Sabathia. The left-hander will be 41 years old this season, and with only 12 major league starts under his belt since the end of 2010 season, his ability to succeed is one of the few remaining questions in this year's rotation.
Pettitte's 2012 consisted of a relatively high number of strikeouts and ground balls based on his career numbers. From 2010 to 2012, his K% jumped 4 points, and his ground ball rate increased by nearly 13%. So how did he achieve these results?
Despite a slight loss in velocity, Pettitte's fastballs had better movement in 2012. Both his four-seam fastball and cutter had around 2 inches of additional vertical "rising" action, compared to 2010, while his sinker gained an additional inch of rise and horizontal movement towards left-handed hitters. The improved movement on the four-seam and cutter correlates with some additional strikeouts, a higher foul rate, and a higher called strike rate. Additionally, his fly ball rates slightly increased, while the line drive rates decreased. The sinker saw the most improvement, as its ground ball rate increased from around 14% to 18%, and he only allowed 3 total fly balls on the sinker the entire year.
The response from his breaking pitches were similar. Overall, the slider, curveball, and changeup all saw around 2 inches of additional rising action, while they maintained the same horizontal movement. The slider continued to be his most dependable out pitch, which he used around a quarter of the time. It achieved almost a 22% whiff rate, a 1% increase from 2010. The curveball and changeup saw the best improvement, as the curveball's whiff rate jumped from 7% to 9%, and the changeup increased from 10% to 14%. With a better changeup, Pettitte was able to decrease his platoon split from a .343 wOBA against righties in 2010, to a .301 in 2012.
All of Pettitte's pitches saw an increase in vertical movement, likely because he was able to increase the spin rate on all six pitches. Though you'd expect higher fly ball rates from an overall increase in rising action, the overall movement on the sinker and a bump in usage caused a dramatic increase in his ground ball rates. The four-seam fastball and cutter also became set up pitches that had more control and were harder for hitters to square up. While the slider remained close to the same, the curveball and changeup saw increased movement that likely contributed to more strikeouts, and thus a better a better platoon advantage.
In the end, Pettitte's 2012 results look very real, despite the small sample size. Assuming he can continue to put the higher spin rate on his pitches, he should have no problem replicating the results of his strikeouts and walks. I'm less certain about the batted ball rates, since there is such an extreme difference between 2010 and 2012, though the PITCHf/x numbers indicate that there was nothing out of the ordinary in 2012. In terms of luck, we'll likely see his left on base rate decrease, and with a weaker left side defense on the infield, he'll probably see his ERA much closer to his 2012 3.48 FIP, rather than his 2.87 ERA.