Keller was optioned to Triple-A Indianapolis on Wednesday.
With Trevor Williams (side) returning from the injured list Wednesday, Keller is no longer needed in the big-league rotation. The 23-year-old prospect has struggled in a trio of starts for the Pirates this season, posting a 10.50 ERA, 2.25 WHIP and 15:6 K:BB in 12 innings. Keller owns a solid 3.10 ERA and 74:25 K:BB in 58 innings with Triple-A Indianapolis this season, so he should get another shot with the big club later this season.