Red Sox manager John Farrell said that Koji Uehara's pectoral strain is "a unique injury for a pitcher."
Koji Uehara (pectoral) is undergoing an MRI.
Red Sox placed RHP Koji Uehara on the 15-day disabled list with a right pectoral strain.
Koji Uehara has been diagnosed with a strained right pectoral muscle.
Koji Uehara left Tuesday's game against the Giants with an apparent right shoulder injury.
Koji Uehara tossed a 1-2-3 ninth inning Friday against the Yankees for his fifth save of the season.
Koji Uehara picked up his fourth save Saturday, pitching a scoreless inning in a win over the Rays.
Red Sox manager John Farrell said Koji Uehara will serve as the team's closer while Craig Kimbrel is out with a knee injury.
Koji Uehara gave up a home run to Evan Longoria in the ninth inning on Friday, but held on for the save to preserve a 6-5 victory over the Rays.
Koji Uehara threw a perfect inning in a minor league game Monday at Red Sox camp.
Koji Uehara is being slowed by general soreness.
Koji Uehara faced live hitters Thursday for the first time since his August arm injury.
Koji Uehara said the wrist fracture he suffered last season is not affecting him.
Red Sox team president Dave Dombrowski said Friday that Koji Uehara will move to an eighth inning role with Friday's addition of closer Craig Kimbrel.