Keller will start Sunday's game against the Phillies, Jake Crouse of MLB.com reports.
Keller was promoted from Triple-A Indianapolis on Friday and will rejoin Pittsburgh's rotation two days later. The 25-year-old struggled with a 7.04 ERA in 12 starts before being demoted in mid-June to Triple-A, where he made eight starts and posted a 3.21 ERA, 1.43 WHIP and 39:13 K:BB across 28 innings.