Back in 2007 and 2008, Andy Sonnanstine used to be a sabermetric darling. In his first two seasons in the majors, Sonny was an effective starter whose talent didn't seem to be captured by his ERA. He posted a 5.85 ERA in 130 innings in 2007 and a 4.38 ERA in 193 innings in 2008, but both those years his peripheral statistics suggested he could do even better than that. His strikeout rates were never anything mindblowing (around 6 K/9), but Sonny kept his walk rate so low (1.7 BB/9) and wasn't especially "hittable" (.302 BABIP in 2008), so he looked like an average to above-average starter.
Since then, things haven't gone quite so well.
Due to Jeff Niemann and David Price pushing their way onto the 2009 ballclub, one of the Rays' existing starters needed to move to the bullpen or be traded. Since the previous long man in the bullpen, Jason Hammel, had just been traded to the Rockies, there was an open spot waiting for some of the Rays' starting pitching depth. Although posting good numbers in 2008, Andy seemed the most likely choice: he had the most marginal stuff of any of the Rays' pitchers, and he was more flexible and less of an injury risk than Jeff Niemann. Andy got moved to the bullpen, and the Rays haven't looked back.
Most pitchers improve when moving to the bullpen, as they can "crank it up" instead of pacing themselves over multiple innings and can abandon their weakest pitches, but that hasn't been the case with Sonny. Since moving to the bullpen, Sonny's talent has seemingly deteriorated: he's now walking close to three batters per nine and his strikeout rate has dropped a tiny bit as well. He's letting up a higher percentage of homeruns and no matter how you look at it, he's become a middling reliever. He posted a 6.77 ERA / 5.45 FIP in 2009 and a 4.44 ERA / 4.87 FIP in 2010. He's a fine low leverage, mop up pitcher, but that seems to be his ceiling at this point.
Right now, it's easy to look back in hindsight and second guess. Most pitchers succeed better in the bullpen because they can increase their velocity and focus on their strongest pitches, but Sonny's strength never came from velocity or plus pitches: his arsenal is mediocre and he thrived by locating his pitches exactly. It may be that this sort of skill set is ill suited for transitioning to the bullpen, as it's possibly more difficult to maintain such impeccable control when pitching on an irregular schedule. How many relievers out there only walk 1-2 batters per nine?* Last season, there were only 14 relievers (out of a total of 207) that pitched more than 30 innings, yet had a walk rate below 2 BB/9.
*Granted, this sort of analysis is obviously faulty. Relievers are put in the bullpen because they're not good enough to be starting pitchers, so poor command may not be a result of pitching in the bullpen, but may simply be a product of the pitchers that get placed in the bullpen. I'd be interested to see some numbers crunched in this regard.
So what should we expect from Andy in 2011? If Marcels is to be believed (Marcels is a very simplistic projection system but still rather reliable), Andy should increase his strikeouts and decrease his walks slightly... yet still post a 4.65 ERA / 4.51 FIP. He's an adequate mop up man and can fill in for some spot starts, but his career has definitely gone downhill. He's no longer the pitcher he once was, and it's unlikely at this point that he ever regains that summit.
Earlier this spring, Sonny let up a MLB record 5 homeruns in one Spring Training start. He had a couple rough starts to begin the spring, but he did just pitch four scoreless innings against the Red Sox in his most recent outing. He's been working on getting his mechanics in line, so I wouldn't worry about Sonny at this point; he is what he is, and he'll be a fine long man out of the 'pen for the Rays. Just don't expect too much more.