Lopes staggered away from the plate after getting beaned by a 91 mph fastball from Detroit Tigers starter Drew VerHagen in the fourth inning. The rookie initially stayed in the game but was replaced by Dylan Moore in the seventh.
Lopes appeared to duck his head at the last instant to prevent the ball from hitting him in the jaw. He was checked by a trainer for a couple of minutes, then walked slowly to first base.
The team said Lopes is in the concussion protocol.
VerHagen received hearty boos from the crowd when he attempted to pick off Lopes at first a few pitches later. Lopes countered by stealing second, and he scored on J.P. Crawford's triple. Seattle won 10-2.
Lopes, 25, is getting his chance on the big league roster because of Dee Gordon's trip to the 10-day injured list with a strained quadriceps.