Perez aggravated his intercostal injury in Thursday's game against the Indians, Rustin Dodd of The Kansas City Star reports.
Perez recorded two hits in three at-bats Thursday, but he was not able to finish the game and was spotted with a heavy wrap on his side afterward. Perez missed a couple weeks with the same injury earlier this year, so if it is the similar event this time around, it could cost him the rest of the regular season.