Gardenhire was standing at first base at Comerica Park before the game when he was struck on the ear by a ball thrown by Twins infielder Alexi Casilla, the Minneapolis Star Tribune reported.
The newspaper said Gardenhire's ear was bleeding and the Twins' skipper was suffering from a headache afterward.
Twins officials indicated he would continue to be monitored by doctors. He remained in the clubhouse to start the game.
Jeb Bush 'thinking about running for president' in 2016
Avril Lavigne denies racism in 'Hello Kitty' music video