How good would Andy Pettitte be if he came back?

Whatever decision Andy Pettitte ends up making, things will get mushy around here when he makes it. If he retires, anticipate a lot of gushing posts about the big lefty and his great career. Should he come back for another season, expect just as many posts trumpeting his return and the impact it will have on the 2011 Yankees. (My official stance is that he definitely has something left in the tank, and should absolutely, unequivocally come back for another season, without question, by the way.)

With that in mind, I had wanted to keep this post on the back-burner until Pettitte had made his decision. Unfortunately, I began the analysis and developed an itchy-trigger finger. Plus, there’s not a lot of content to produce right now. Larry’s writing about Brian Fuentes and Bartolo Colon, for crying out loud! (Editor’s Note: Ha! Sad, but true.)

As with my other recent posts projecting the 2011 season for a variety of players, this post uses Baseball-Reference’s similarity scores to determine which player at the same age as Andy had the most similar career to him. Andy’s recent performance, combined with the age 39 season of the player he has been most similar to through his career to date, offers a strong tool for informing an educated guess about what Pettitte would bring, should he decide to come back for an age 39 season. There is more science behind this methodology than meets the eye. Most projection systems are heavily weighted towards using a player’s age, recent performance, and the performance of similar players at the same ages. While the methodology here stops short of trying to pinpoint, say, Andy’s 2011 ERA, it uses all the same ingredients as PECOTA or CAIRO.

The reason I couldn’t help but write this post before Andy had made his announcement is because I knew which player he was going to be most similar to before I’d even looked at the numbers. The player is obvious. Every year since his age 35 season, Andy Pettitte has had a similarity score above 900 (1000 is as high as it goes) when compared to only one player: Mike Mussina.

There are two problems with Andy rating similar to Mike Mussina. The first is simple: In his age 39 season, which was his last in baseball, Moose tossed 200.1 innings of 132 ERA+ baseball. That was the season that Moose learned to harness his secondary pitches and get batters out despite diminished velocity, something similar to what we’ve seen from Andy Pettitte in recent years, who posted a 130 ERA+ last year in an injury-shortened season. If Pettitte really is that similar to Moose — and the comparative numbers are obvious — then he probably has a lot left in the tank, making a retirement decision that much harder to accept.

The second problem with Pettitte comparing so favorably to Mussina is symbolic. Moose hung ‘em up early. Prior to Mussina calling it quits after his best season since he was 34 I just assumed that all athletes hung around until the collective owners and GMs of their sports were no longer willing to pay for their services. From Michael Jordan to Joe Montana to Roger Clemens, successful athletes all did the same thing: Sure, they toyed with the idea of retirement, but they all kept going until their bodies couldn’t deliver any more — all except Mussina, that is. Moose called it quits after one of his best seasons as a pro. His decision hurt the Yankees, the results of the 2009 season aside. The team is perpetually short a starter or two. As a superstitious sports fan I find it unsettling to see that Andy Pettitte and Moose rate as similar on the field, because they were already similar off the field.

After Moose, the similarity scores for Pettitte fall from 918 to a range in the 850s and 860s. His comparables, in order, after Moose, are Jack Morris, Kevin Brown, Tommy John and Tom Glavine. Of them, only Morris was ineffective his age 39 season. Brown suffered from an injury (shocking!), but still posted an ERA+ of 110 (with the Yankees, no less) while John and Glavine each tossed more than 210 innings of 108 or better ERA+ baseball.

Put another way, the computer thinks the odds are far more likely than not that Pettitte would have an excellent season if he were to come back. For all our sakes, let’s hope he does. I, for one, am not ready to see him call it a career.