The move is retroactive to Tuesday and followed a bullpen session by Buchholz before Wednesday night's game against the Atlanta Braves.
"We had a chance to sit down and talk to him before he went out on the field," manager John Farrell said before knowing the results of the session, "but we're going to be cautious. We don't want to put him at risk of suffering any other type of injury, whether that's to the knee or shows up elsewhere. We've got to be careful."
The Red Sox recalled right-hander Alex Wilson from Triple-A Pawtucket to fill Buchholz's roster spot.
On Monday, Buchholz walked a career-high eight while allowing six runs in three-plus innings of an 8-6 win at Atlanta that ended Boston's 10-game losing streak.
On Tuesday, Farrell said Buchholz continued pitching Monday after his left knee buckled. He said Buchholz initially denied that happened when catcher A.J. Pierzynski went to the mound.
Buchholz is 2-4 with a 7.02 ERA in 10 starts. Last year he was 12-1 with a 1.74 ERA in 16 starts and made the AL All-Star team for the first time. In four postseason starts, he was 0-0 with a 4.35 ERA as Boston won the World Series. He had missed three months with a neck injury before coming off the disabled list on Sept. 10.
Wilson pitched one inning in relief in two previous stints with Boston this season. At Pawtucket, he was 1-0 with five saves and a 1.42 ERA in 18 relief appearances. He made his major league debut last year, going 1-1 with a 4.88 ERA with Boston.
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