Grimm was ineffective for the Royals earlier this season, posting a 21.86 ERA in 10 appearances.
Grimm's struggles continued at Triple-A, where in nine outings he had an 8.68 ERA. But Grimm was near the end of his 30-day rehab assignment and he had to be reinstated.
Royals manager Ned Yost said he didn't have a defined role for Grimm upon his return. He started the season as a setup man to closer Kelvin Herrera.
"He's going to pitch out of the 'pen," Yost said, "and that's all I know."
Bonifacio to start rehab Royals outfielder Jorge Bonifacio, serving an 80-game suspension for testing positive for a performance-enhancing substance, will begin a 15-day rehab assignment on Wednesday at Omaha.
According to club officials, Bonifacio must be reinstated from the suspended list after the Royals-Brewers game on June 27. A corresponding move on the 40-man roster (if need be) or the 25-man roster likely would be made the next day.
Yost said he did not know if Bonifacio would then be optioned to Omaha or would join the big league club.