Pitcher Michael Pineda, who the Yankees acquired in the offseason for top prospect Jesus Montero in the offseason, has been diagnosed as having a torn labrum in his right (pitching) shoulder.

Yankees general manager Brian Cashman told reporters that Pineda was healthy when he was acquired from Seattle, and that he believed the injury occurred last week, in the pitcher’s final rehab outing. Pineda was tested after the 15-pitch outing, showing significantly different results than had been uncovered prior to the appearance.

Surgery on the 23-year-old is expected to be performed Tuesday.

Pineda is coming off a rookie season in 2011 in which he made the American League All-Star team, finishing at 9-10 with a 3.74 ERA. Montero has played in 14 games with the Mariners (10 at designated hitter, four at catcher), hitting .254 with two home runs and a .643 OPS.