While trying to beat out a play at first, he over extended and came up gimpy. He stayed in the game, but when running to second looked like he was running on it gingerly. He then did not re-enter to play defense in the next half inning.
Maybe it's the cynic in me, but when I hear "knee sprain" I think "torn ACL". Judging by his reaction and that he didn't immediately leave, I doubt anything is "torn."