Frelick fouled a ball off his right knee in the fifth inning during the second game of Thursday's doubleheader for Triple-A Nashville, walking off the field with a limp, Curt Hogg of the Milwaukee Journal Sentinel reports.
Given that he did not remain in the game, it's possible that the injury proves to be worse than just a bruise. Frelick had hit .308 with one home run and two steals in seven Triple-A games since returning from a thumb injury.