Keller landed on the minor-league DL at High-A Bradenton with a back strain.
He had been wowing scouts again this season after adding a changeup to his arsenal. Keller, 21, posted a 2.93 ERA, 0.93 WHIP and 39:8 K:BB in 40 innings prior to the injury. He should be able to return and re-establish his value as a top-15 pitching prospect sometime this summer.