The left-hander threw a cutter up and in to left-handed hitter Michael Brantley, then raised his hand to signal for the trainer.
Miley pointed to his right side as he waited near the mound before walking to the dugout after 19 pitches. He was replaced by reliever Brent Suter.
Miley (1-0) was making just his second start for the Brewers after winning his debut Wednesday against Cincinnati. Signed to a minor league contract in the offseason, Miley was on pace to make the rotation out of spring training before being sidelined by a groin injury.