Matusz has pitched in 68 major league games, all of them starts. The Orioles had him serve as a reliever in four games recently with Norfolk, and he did well enough to warrant a look at the big league level.
Manager Buck Showalter said, "We'll see if it plays up here."
Matusz will be the only lefty in the bullpen. He replaces J.C. Romero, who was designated for assignment on Friday. Romero had a 6.75 ERA in five appearances with the Orioles.