To date, Pettitte is 3-0 with a 1.29 ERA, 1.07 WHIP, and 22 K in 28 IP - fantastic numbers, to say the least. Pettitte has been one of my favorite players for as long as I've been watching baseball, but deep down I know that he cannot sustain ace-like production... he's still a fine starter, but not on this level. Let the digging begin.
I noticed that Pettitte's pitch selection has changed a bit from 2009. Through four starts Pettitte has thrown his curveball 16.8% of the time and his change-up 5.4%. In 2009, those numbers were 13.4% and 9.4%, respectively. Like 2009, his curve has been very effective whereas his change has not. However, his utilization of the two has shifted noticeably, which has certainly accounted for some of his improvement. Pettitte has not used his curve as much since his last stint with the Yankees - perhaps this renewed commitment to this effective pitch will continue to help him thrive.
Now for the bad news - Pettitte's batted ball data and left on-base numbers.
First, Pettitte has yet to allow a home run. While some of this can be attributed to him pounding the lower half of the zone, utilizing a more effective breaking ball, and garnering weak contact, it's nearly impossible for such a trend to continue. Pettitte's allowing 30.3% flyballs, which is right in-line with his career mark. Eventually, balls will start leaving the park - his xFIP, a fine 3.73, accounts for this... and leaves me believing that he'll remain quite effective, to boot.
Second, Pettitte's BABIP is .269, which appears unsustainable. His career BABIP is .315 and his xBABIP is .316. Again, more effective pitches could lead to a lower BABIP... but some regression should be expected.
Finally, through four starts Pettitte is stranding an astounding 87.1% of baserunners. Aside from his career year in 2005 (when he stranded 79.7%), he has never stranded more than 74% of baserunners. His career rate is 71.4% and league-average fluctuates between 70% and 72%. Regression should be expected once more. On the optimistic side, however, this is also accounted for in xFIP - and 3.73 is a fine number.
Overall, I do believe that Pettitte is set to have his most effective year since returning to the Yankees. While the sample sizes remain small, his curve is more effective than it has been in several years, he's striking out more and walking fewer batter compared to the last couple of years, and the defense behind him is better than it has been in years. While I can say with near certainty that he cannot continue to pitch like an ace, I do think that it's reasonable to expect him to continue to improve over his last few seasons.
