"That I don't know at this point," Red Sox manager John Farrell said of what caused the injury. "The one play, I don't know if he jarred it when he went down to catch himself or brace himself on the errant throw that he kept in front of him. Those are things that, it's inconclusive at this point.
"He took a funny swing in the last at-bat on the strikeout, whether that's a contributing factor, I don't know. The spasm speaks for itself, and he had to come out."
Wednesday marked the first time Ramirez had played first base since Game 3 of last October's American League Division Series against the Indians. Playing under National League rules, the Red Sox wanted Ramirez's bat in the lineup on Wednesday.
Mitch Moreland, who replaced Ramirez in Wednesday's game, will start Thursday's finale at first base after being out of the lineup for the first time all season. The Red Sox don't know yet if Ramirez will be available to serve as a pinch-hitter Thursday, the finale of the club's six-game road trip.
Boston gets the designated hitter back with a three-game series against the Rays at Fenway Park this weekend, but they head to St. Louis for a pair of games next week.
Ramirez went 0-for-2 with a strikeout and left four runners on base before his exit.
Ian Browne has covered the Red Sox for MLB.com since 2002. Follow him on Twitter @IanMBrowne and Facebook. This story was not subject to the approval of Major League Baseball or its clubs.