"Clay has felt a little irritation in the collarbone/AC joint area," said Red Sox manager John Farrell. "This is not related to any other part of the shoulder. It's precautionary more than anything. We have decided to bump him, push him back. And Alfredo will start [Monday]."
Buchholz said that the decision was his, and emphasized he would make the start as scheduled if it was in the middle of a pennant race.
"But it's not," Buchholz said. "Being as early in the season as it is, I want to make sure I'm 100 percent every time I go out there rather than 80, 85 percent and risk having this be something that's going to linger. I just want to cut it right now and hopefully in a couple more days, I'll be back where I was at."
Farrell said Buchholz might not be skipped entirely and could pitch later in the week.
In Buchholz's last start on May 22, he beat the White Sox in Chicago. It was the next day he started to feel something.
"I slept on my shoulder," Buchholz said. "I normally don't do that. For some reason I did. The night we got back from the road trip, got back in bed, had my little girl with me, and I fell asleep like that instead of on my back like I usually do. I could pitch [Monday] if I had to but I don't have to. I'll take my time and get it all the way out."
Buchholz has been one of the game's best pitchers this season, going 7-0 with a 1.73 ERA in 10 starts.
"That's what's hard about it," said Buchholz. "I want to be out there every day. I want to be out there every day, but I don't think I would be doing the team justice if I'm not 100 percent."
Aceves has started three times for the Red Sox this season, going 1-1 with an 8.10 ERA.
Ian Browne is a reporter for MLB.com. Read his blog, Brownie Points, and follow him on Twitter @IanMBrowne. This story was not subject to the approval of Major League Baseball or its clubs.