Diamondbacks manager Torey Lovullo said Mejia would be called up from Triple-A Reno to start Monday's game against Atlanta at Chase Field, the Associated Press reports.
Mejia will join Arizona's six-man rotation as a replacement for rookie Tyler Gilbert (elbow), who was placed on the 10-day injured list Wednesday. The 24-year-old right-hander previously made a pair of starts for Arizona in late August, covering 11 innings between the outings and allowing six runs on 14 hits and three walks. He'll tentatively line up for a two-start week, with his second turn coming Sunday at home versus the Dodgers.