Jason Hammel's had a fairly rocky (heh) career, spending three years each with Tampa Bay and Colorado before being acquired by the Orioles (along with Matt Lindstrom) for Jeremy Guthrie shortly before the 2012 season. That trade turned out to be a brilliant one, as Guthrie struggled at Coors Field, Lindstrom was a solid reliever, and Hammel's career year made him the ace of the O's first winning and playoff team in fifteen years. Coming into 2013, most would have agreed that regardless of how much Hammel regressed, the trade would still look good. That's fortunate, because Hammel regressed quite a bit, his ERA jumping from 3.43 to 4.97.
It's hard to say exactly what caused Hammel's struggles, but we can point to a number of factors. For one, his strikeout rate dropped dramatically. In 2012, he struck out 8.62 batters per nine innings, or 22.9% of those he faced. In 2013, those numbers plummeted to 6.20 K/9 and 15.7 K%. His groundball rate also fell sharply, from 53.2% to 40.1%. That, plus a slight increase in his HR/FB rate, doubled his HR/9 (0.69 in 2012, 1.42 in 2013). In short, in pretty much every go-to component stat (except for walk rate), Hammel went from career-bests in 2012 to career-worsts in 2013.
Obviously, when all those peripheral stats get worse, it's no surprise when ERA jumps. But it's hard to understand exactly what happened to Hammel between the two seasons. PITCHf/x suggests that the movement on his pitches was comparable, though his velocity dipped by about 1 mph across the board. Simply watching him this season, his "stuff" didn't appear to be worse, but batters made a lot more contact: 84.5% of the time in '13 versus 77.3% in '12. Fortunately, the eye test can help us a bit here, as Hammel did seem to struggle with his command a lot more this year than last. In addition, lefties crushed him this season (.381 wOBA) after hardly being able to touch him in 2012 (.262 wOBA). Most pitchers struggle a bit against opposite-handed batters, so it's no surprise that he wasn't able to repeat '12 against left-handers, but no one expected him to catch the Brian Matusz/Tommy Hunter platoon-split disease.
Hammel hits free agency this offseason, and it's hard to see the Orioles going after him at all, especially if they're as interested in re-signing Scott Feldman as has been rumored. If the Warehouse wants to stabilize the rotation, a guy like Hammel is perhaps the worst way to do so: significantly more expensive than, say, Zach Britton, without being as dependable as, say, Scott Feldman. Hammel seems most likely to get an incentive-laden one-year deal from someone, but it probably won't be from Baltimore.