Only, Pena was pretty awful last season. His 5.10 ERA came with an ugly 1.24 K/BB ratio that's well under his career average. Prior to the 2010 season, his career ERA was a very average 4.02, with three solid seasons dealing in a hitter's park. Obviously, you wouldn't expect an average reliever to be much of anything as a starter, but "not much of anything" might still clear Phil Humber's upside.
Also obviously, that doesn't resolve Pena's very ugly 2010. The open question is whether or not that sustained bout of suck represents a new, worse level of performance going forward. If so, the Sox probably should have just pocketed the $1.6M they doled out for his second arbitration-eligible season and non-tendered him instead. I guess that means they have some faith in him bouncing back. Or maybe he was more like a 25 cent gumball after a big dinner out: screw it, why not?
So what changed? Pitch f/x suggests not much.
He lost a tick on his fastball, but 94 is still plenty healthy. The movement on the pitch is nothing special, but there's nothing there that says "implosion". Same with the slider. It's a bit slower, but he added more downward bite to the pitch. To put that in some context:
This here heat map shows pitch effectiveness by movement. The warmer on the spectrum, the more effective the pitch. This particular heat map is for sliders to same-handed batters and clicking through to the big map shows that Pena's slider is pretty solidly in an effective range. And keep in mind this doesn't control for fastball speed. Presumably, the harder you throw, the better your slider is going to be.
And in seasons prior to 2010, Pena's slider has been ridiculous. In 2009, for example, batters swung and missed at the pitch at a 25% clip, way above the 13% average for all sliders, but right around Pena's career average for the pitch. In 2010, that dropped to 11% despite no huge difference in movement or speed.
I think that makes the culprit very likely faltering command. Pena threw fewer strikes per pitch with both his fastball and slider in 2010, putting him in bad counts with far more frequency. The other suggestive anomaly: all those change-ups thrown. It wasn't a result of facing more lefties. He simply chose to throw far more. Along with the walk rate bump, I think it's reasonable to suggest Pena either lost some of his feel for the slider or was perhaps being inconsistent in his mechanics.
Bolstering that conclusion further, Pena got nearly his usual number of whiffs in strikeout counts with the slider. So when he only needed to be in the neighborhood with the pitch, his results were solid. When he had to demonstrate real command, he faltered. Which led to compensation: throwing more changes and fastballs.
Unless there's some suggestion that he was injured, I don't see why he couldn't correct the problem with Coop's help. At the same time, his season was long enough to suggest a sustained difficulty that might not remedy itself until some possibly lengthy duration. If you accept that pitchers are helped by having defined roles, then Pena working as a starter is some cause for concern. If you think repetitions are what are needed to fix these kinds of problems, maybe it's for the better. Either way, he needs some work to get back to his previous levels of effectiveness. He certainly hasn't demonstrated that he's prepared to make the leap from blah reliever to effective starter. Rather, he's showing symptoms that get pitchers shunted to the pen in the first place.
*He's 28 with 2 career MLB starts. Pena has one more and got them all last year.