Buchholz threw a bullpen session before Tuesday night's game against the Tampa Bay Rays, but apparently did not show sufficient progress to clear him to start this weekend.
"We didn't feel like we had enough time to give him the proper amount of work to prepare for that start," Boston manager John Farrell said. "We didn't want to take any chances."
Buchholz will not be eligible to pitch until June 25, when the Sox play the Colorado Rockies in Fenway Park.
The Red Sox also placed catcher David Ross on the seven-day concussion disabled list. Ross was placed on the list back on May 12, and missed 11 games before being activated May 24. Ross was hit by another foul ball in the third inning of Friday night's game against the Baltimore Orioles. Though he finished the game, he hasn't played since.
"He's still having symptoms with the concussion," Farrell said. "The foul tip the other day in Baltimore kind of brought some of those symptoms back."
The Red Sox will need another starter Saturday in Detroit. The most likely candidate for a call-up at this stage would appear to be Allen Webster, who pitched one-hit ball over six scoreless innings against Buffalo in his most recent start Friday and wasn't slated to start again until Thursday.
The Sox have not announced rotation plans past Wednesday, when Ryan Dempster is scheduled to pitch the series finale against the Rays. John Lackey, coming off his strong start in Baltimore on Saturday (7 IP, 2 ER), figures to stay on regular rest and pitch the series opener in Detroit on Thursday night. It's conceivable that Farrell could pitch Webster on six days' rest Friday and give Jon Lester, who pitched Sunday, an extra day of rest and pitch him Saturday.