Matsuzaka sustained the injury in the fourth inning of Tuesday's game at Seattle. He left the game with what was described at the time as a tired shoulder.
Before Boston's game at Baltimore Friday, the team said on its Web site that an MRI in Boston revealed no structural damage.
"There were no structural changes, which is really good news," Red Sox Manager Terry Francona said. "The final diagnosis is a mild deltoid rotator cuff strain. The best way we can see to attack this is to shut him down to strengthen the arm and get him ready to pitch the rest of the season."
To date, the native of Japan is 8-0 with an ERA of 2.53. Matsuzaka has walked 38 batters and struck out 55 in 64 innings (11 starts).
His roster spot will be filled by first baseman-outfielder Jeff Bailey.
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