Buchholz (9-0) did not throw a bullpen session Saturday, although he did do some flat-ground work. He's been on the disabled list since June 9.
"It's likely that at some point we're probably going to have to get him out on a rehab assignment - at least one start for now," manager John Farrell said. "But there's no timeframe for that. We've got to get him on the mound first and then build from there."
The team had hoped Buchholz might be able to pitch Tuesday, but now Ryan Dempster is scheduled to make that start against Colorado.
"It's just not quite to the point where I can get on the mound right now," Buchholz said. "I'm still trying to feel it out, I guess. It's taking a bit longer than I want it to."
Boston recalled right-hander Allen Webster from Triple-A Pawtucket to start Saturday night against Detroit.
When healthy this year, Buchholz has been perhaps the best pitcher in the American League. He leads the majors with a 1.71 ERA, but he made an early exit from his latest start against the Los Angeles Angels on June 8.
The Red Sox later put him on the DL.
"Moving forward, just not as quick a pace as I'd like," Buchholz said. "If it's time that needs to help it heal then we've just got to give it a little more than we were expecting."