Harrison underwent an MRI after Saturday's start against the Los Angeles Angels, which revealed an inflamed nerve in his lower back.
Harrison, whose DL stint is retroactive to April 7, is 0-2 with an 8.44 ERA in two starts. He had a epidural injection in his lower back and didn't make the road trip.
Grimm won his only start so far this season for Triple-A Round Rock, allowing a run in 5 2/3 innings. The right-hander had four strikeouts and two walks.
in another move, right-handed pitcher Justin Miller was recalled from Double-A Frisco and placed on the 60-day disabled list. He is rehabbing from Tommy John surgery. The move leaves the Rangers with 39 players on their 40-man major league roster.