The move comes one day after the right-hander exited a 4-0 win against the San Francisco Giants, clutching his right arm after throwing only seven pitches.
Manager John Farrell said Wednesday night that an MRI confirmed that Uehara indeed suffered a strain of the right pectoral muscle.
"It obviously confirms a strain," Farrell said of the MRI. "To what extent, we're still getting our arms around that. This is a unique injury for a pitcher, so the best thing I can tell you is the MRI does confirm the strain." .
Before the game, Farrell confirmed that newly acquired reliever Brad Ziegler will assume closer duties. Ziegler, a 36-year-old right-handed sidearmer, had 18 saves this season for the Arizona Diamondbacks before coming to Boston in a July 9 trade. He threw an inning of scoreless relief to finish Boston's 11-7 win against the Giants on Wednesday night.
"There may be some left-handed situations, if we're facing a string of left-handers, that there could be other options in the bullpen to close out. I don't want to rule that out," Farrell said. "But as far as building through the game and working back to a closer, it's Brad in that spot."
The Red Sox are still without regular closer Craig Kimbrel, who had knee surgery July 11.
Kimbrel was out on the field Wednesday afternoon throwing from flat ground from 90 feet. His initial timetable to return was three to six weeks.
"He's feeling pretty darn good," Farrell said. "He had a very positive exam yesterday, recheck with Dr. [Peter] Asnis, and we're hopeful he gets out to 100-120 feet in the next couple of days as far as his flat-ground catch goes. That next step after that flat-ground work, hopefully that's in the coming days."
In place of Uehara, righty Noe Ramirez has been recalled from Triple-A Pawtucket. In 11 appearances for the Red Sox this season, Ramirez has a 6.55 ERA.