"When he got on the mound and started to really get into his bullpen, he still feels some restriction in that same area," manager John Farrell said. "So he's scheduled for an MRI later today and we'll have more information following that."
After the Red Sox's 5-3 win over the Rockies, Farrell said the results of the MRI were not yet available. When Buchholz could return to the rotation remains unclear, but he won't rule out a return after the All-Star break.
"I mean, I hope not," Buchholz said. "Hopefully we can kind of take care of this real quick and get the process moving quicker than it has been. Still not ruling that out. Definitely don't want that to happen. I want to pitch before the break. I don't want it to be all this time off and then come back from the break with four days added on to that. I want to figure it out and get on the mound and do whatever I need to do well."
Farrell had said Tuesday that he hoped Buchholz could make a rehab start as early as Sunday, but that is now unlikely.
Before Buchholz can return to action, he will need to throw two bullpen sessions without any issues. The Sox hoped Wednesday would be the first of two 'pen sessions, but after throwing pain-free on flat ground, Buchholz experienced problems throwing from the mound.
"It's just not quite there yet," Buchholz said. "Playing long toss, playing catch it feels fine. It's just that incline right now. It's not something that's a terrible feeling, it's just I want it to be gone. I don't feel like pitching, going out some starts and then having it flare up again."
When healthy, Buchholz has been dominant this season, posting a 9-0 record with a 1.71 ERA, but health has been a continuing issue for him. He missed a start in late May due to problems with his AC joint and injured his neck when making an awkward throw to first base against the Angels on June 8.
"What he's meant for us -- I don't know our overall team record, but knowing that he's 9-0 himself and the number of games that we've won when he's been on the mound -- you know his performance was as good as you're going to find in baseball for the time that he's been active," Farrell said. "Top-of-the-rotation starting pitchers, when they're absent, are hard to replace."
Michael Periatt is an associate reporter for MLB.com. Follow him on Twitter @Michael Periatt.