In a related move, the team optioned right-hander Casey Sadler to AAA.
Locke will get the nod Monday against the San Francisco Giants, his first big-league start since Sept. 22.
The team is still hoping left-hander Wandy Rodriguez can return from right knee inflammation. The Pirates want him to make at least one more rehab start in the minors.
Last season, Locke made the All-Star team after starting the season with a 9-2 record and a 2.11 ERA. But over his last six starts, Locke's ERA ballooned to 8.11 and opposing batters he faced hit nearly .390. His struggles earned him a demotion to Double-A.SI.COM STAFF: MLB Power Rankings: Athletics in first, but Marlins are rolling