Heilman's an interesting case - an effective and occasionally dominant reliever between 2005-2007, 2008 saw him take a substantial step backwards, as his line drive rate increased, his strike rate dropped a few ticks, and his walk rate more than doubled. His ERA shot up as a result, and in a hurry he went from being a highly-touted pitcher with promise to a favorite target for criticism. Since traded, many Mets fans who've grown sick of his act will tell you "he's your problem now," as Aaron Heilman today just doesn't seem nearly as exciting as Aaron Heilman a year ago.
So what happened in 2008 that changed a good reliever into a mediocre one? A lot of people have made mention of the fact that Heilman started throwing a slider last year, a pitch he all but abandoned seasons before. He went from throwing ~1% sliders in 06/07 to ~11% sliders in 08, and with everything else more or less staying consistent, this seems like the easiest and most obvious thing to blame. According to this theory, Heilman adding a slider caused him to throw more pitches out of the zone, which got everything all bollocksed up.
It's tempting, then, to suggest that Heilman was just throwing a wild slider, and that that was the whole problem. And there's even a little truth in there, too - based on my best attempts at PITCHf/x pitch identity classification, only 56% of Heilman's sliders last year went for strikes. That's a low percentage, and surely accounts for a fraction of Heilman's trouble.
But it doesn't account for all of it. No, to explain the whole thing, we need to look at the rest of his pitches as well. Turns out the slider wasn't the only pitch with which Heilman was having a little difficulty last year.
* based on a limited but presumably representative sample of his pitches
Look at that drop in fastball strike%. With the expressed caveat that our limited 2007 data sample could be giving us false positives and negatives, that's a huge step back. More than 60% of Aaron Heilman's pitches are fastballs. The difference between 62% and 72% of them going for strikes is unquestionably significant. Somehow last year, Aaron Heilman seems to have lost consistent control of his primary pitch.
It wasn't a movement thing - his average fastball movement in 2008 was almost identical to his average fastball movement in 2007. The pitch itself wasn't significantly different in movement or velocity. It was just different in control and results. Only 62% of his fastballs were strikes, and on top of that, of the 128 fastballs that were put in play, 38 of them (30%) were line drives. The line drive rate on his other two pitches was half that. So not only was Heilman struggling to locate his fastball, but it was also his pitch most frequently getting sent back into the field of play with alarming speed.
(Never mind the changeup - 61% is a low strike rate, but that's Heilman's strikeout pitch, so it's going to spend a lot of its time outside of the zone. Heilman's changeup was awesome last year. That wasn't the problem.)
Aaron Heilman got into a lot of trouble last year, but while that coincided with the rebirth of his shelved slider, it would appear that it was first and foremost his fastball that was letting him down. And that's bad. You never want to see a guy struggling to get good results with the pitch he throws far more often than all his other ones combined.
Whether or not that was related to his slider, I can't say. I certainly think it's possible, if not probable. Adding a pitch to your repertoire can easily alter your throwing motion, and altering your throwing motion can have any number of different effects on your results. So it wouldn't come as a surprise to find out that Aaron Heilman's slider was responsible for the loss in effectiveness of his fastball. Unfortunately, though, this is as far as I can go. As outside observers, we can establish a correlation, but we can't establish causation.
So it's on the Mariners to do that bit for themselves. They need to figure out the status of Heilman's slider, the status of his fastball, what - if any - effect the former has on the latter, and whether or not Heilman can be more comfortable throwing all three pitches in 2009 than he was in 2008. It's the last thing that's the biggie, here, because if 2008 was simply the result of adjustment-related growing pains, then the 'why' for 2008 doesn't matter so long as it's smooth sailing ahead. But this is an important issue to resolve, because the fate of Aaron Heilman's career rests on identifying exactly what went wrong last season, and making sure it doesn't happen again. Heilman may want to start, but unless he fixes what's broken, he'll be lucky to still have a decent job a few years down the road.