The Pirates plan to start Keller in the second game of Monday's doubleheader at the Cincinnati Reds.
"In a perfect world, we would've given him some more time," Pirates general manager Neal Huntington said Sunday. "We're not in a perfect world. We feel like he's ready to come up here and compete."
Keller, 23, was ranked as the No. 24 prospect in all of baseball by ESPN's Keith Law in January. Law ranked him second in the Pirates' system behind third baseman Ke'Bryan Hayes (No. 18 overall).
Pittsburgh selected Keller in the second round of the 2014 draft. He's 5-0 in nine starts with a 3.45 ERA for Triple-A Indianapolis this season.