Valentine was vague in describing Buchholz's health.
"I haven't gotten a lot of information about exactly how it happened or whatever happened," the manager said. "We'll have more later."
Buchholz last pitched on Tuesday vs. the Marlins. He was in the midst of his best stretch of the year with a 4-0 record and 2.43 ERA in his last five starts, including a complete-game shutout vs. the Orioles on June 7.
The right-hander checked into a hospital this week.
"After his last start he was OK the next day and then he started feeling something," Valentine said.
Buchholz is the second member of Boston's rotation to go on the DL in the past nine days. Josh Beckett went on the DL on June 16 with right shoulder inflammation. The move was retroactive to June 12, meaning Beckett could pitch as soon as Wednesday vs. the Blue Jays.
The Red Sox are not in a rush to activate the righty, however.
"We want to make sure that he's right," Valentine said. "He'll have to throw off the mound, feel really good and we'll schedule another bullpen [session] that will then be followed by a start. Hopefully it coincides right when his 15 days are up, but we won't rush it if in fact he needs another day or two."
For now, Boston's rotation consists of Jon Lester, Felix Doubront, Daisuke Matsuzaka, Franklin Morales and Cook.
Matsusaka has made three starts in the Majors this month since recovering from Tommy John surgery. Morales had been a reliever for the last four years before making his first start with Boston last Sunday, followed by another on Saturday. Cook came off the DL Sunday and started vs. the Braves in his second outing of the season.
"It's a work in progress, let's say," Valentine said.