The 24-year-old made a leaping catch into the fence to rob Ryan Braun of a home run but was seen holding the right side of his face as he made his way to the dugout.
He will likely have to pass the league's concussion protocol test in order to return, so if that's the case, he could be done for the season. The Reds wrap up their season with a game at home vs. the Pirates on Sunday.
Hamilton left Wednesday night batting .250 with 56 stolen bases and is among those in the conversation for NL Rookie of the Year. In June, he was named NL Rookie of the Month.