After the 2012 season, Orioles fans had essentially given up on Brian Matusz the starter. Despite 2009 scouting reports describing him as a polished four-pitch lefty with a solid changeup, Matusz has struggled to get righties out for his entire career. Right-handed hitters now have a .300/.370/.490 batting line against him, which is similar to Jose Bautista's performance over the last couple of years. When a pitcher has that large of a platoon split, it's inevitable that he's going to end up in the bullpen, and we saw both Matusz and Tommy Hunter make that transition last year.
There was little question that despite being "stretched out" in spring training, Matusz would return to a bullpen role in 2013. For one thing, he was good at it; he's always been very effective against lefties, holding them to a career line of .205/.264/.354. For perspective, only one qualified batter had a lower OBP in 2013, and only three had overall lines worse than that. Matusz has also struggled in the past with other things that starting pitchers need to do to be successful, such as holding runners and fielding his position.
In his first full year of relief, Matusz was a bit better than I (and, I suspect, most Orioles fans) suspected. He posted a 3.53 ERA, along with a 2.91 FIP (3.51 xFIP). His strikeout rate was the highest of his career, at 8.82 K/9 IP, and his walk rate was his lowest since 2009, at 2.82 BB/9. Those are all good things, but they're largely consequences of Matusz facing more lefties than righties in a season for the first time. Right-handed batters did hit for less power against him this year, slugging only .372, but they still posted a .302 AVG and .375 OBP, which are still far too good for him to be allowed to regularly face righties, especially with men on base.
The '13-'14 offeason is Matusz's second year of arbitration eligibility, but he's under team control through 2016. He's all but certain to get a raise through that process, whether the team makes a deal before the hearings or not, and his $1.6M salary could increase to $3M. That's too much to pay a lefty specialist, but when your lefty specialist has spent the time in the rotation that Matusz has, that kind of salary can be justified in the arbitration process. With lots of other players receiving raises this offseason, there have already been suggestions that Matusz will be traded. If he does remain with the team, he'll supposedly be "stretched out" again in spring training, but I have a hard time seeing him make even an emergency start in the regular season.