PITTSBURGH (AP) -- The Pittsburgh Pirates placed ace A.J. Burnett on the 15-day disabled list with a strained right calf and filled his roster spot by activating right-hander Charlie Morton, who returns to the majors almost a year to the day after having Tommy John surgery.

Morton, who underwent the elbow reconstruction procedure last June 14, will start Thursday night against the San Francisco Giants.

Burnett was scheduled to start Friday against the Dodgers but the 36-year-old notified manager Clint Hurdle of the calf problem Wednesday. He is 4-5 with a 3.12 ERA and is second in the National League with 99 strikeouts. Jeff Locke will start Friday.

The Pirates also released reliever Jose Contreras, who had been on the disabled list with lower back pain.