In order to make room for Jeff Larish and Will Rhymes on the 40-man roster, Billy Buckner and Casey Fien are the odd men out (info via the Tigers' official Twitter).
Since arriving in exchange for Dontrelle Willis from the Diamondbacks, Buckner has performed nothing short of awful in a starting role for the Mud Hens (3-5, 9.40 ERA in eight games).
Casey Fien had already bounced around all over the place during spring training because of a similar situation; a necessary roster move. His last performance in the majors wasn't pretty, though he hasn't been given much of a chance this year.
Good luck to him wherever he winds up, a lot of us at BYB are big fans of his.