The designated hitter underwent an MRI exam before Friday night's game against the Tampa Bay Rays to determine the cause of persistent soreness in his left biceps. The Red Sox were still awaiting results of the test.
"I don't have a return back back in the lineup either," manager John Farrell said.
Ramirez has dealt with sore shoulders for most of the season. The biceps issue is more recent, although Farrell said he "can't say it's not related" to the shoulder problems.
The Red Sox have been waiting for Ramirez to get hot, but since the All-Star break, he's batting only .206 with nine doubles, nine homers, 21 RBIs and a .669 OPS. Overall, Ramirez is batting .238 with 22 homers and a .743 OPS.
"He just feels like he can't trigger," Farrell said. "It's the left shoulder, so as he gets to the point of contact, it's like as he describes it not feeling like he can fire through as he typically does."
Dustin Pedroia served as the designated hitter Friday night, although DH at-bats likely will be split among several players in Ramirez's absence. Right-handed-hitting first baseman Sam Travis also could get more at-bats if Ramirez is out for a while.