He has been hospitalized and needs 10-14 days to recover from intestinal bleeding, the team said.
"We're hoping that we get him back at the end of the 15 days," Valentine said.
According to Valentine, Buchholz was unaffected by the issue during his last start Tuesday at Fenway Park against Miami. It was the first non-quality start in a span of five outings for Buchholz, who had turned a corner after a miserable start to the season. Buchholz began to feel something two days later, Valentine said.
Buchholz is the third member of the opening day rotation to be ushered to the sidelines. Josh Beckett is already on the DL with right shoulder inflammation and Daniel Bard was demoted to Pawtucket to focus on a return to a role in the bullpen. Only Jon Lester and Felix Doubront remain.
Franklin Morales has essentially replaced Beckett and has already made two solid starts. Aaron Cook will be activated to take Buchholz's start Sunday against Atlanta. Cook lasted just 2 2/3 innings in his only start for Boston this year, during which he suffered a serious cut to his left knee and was immediately placed on the DL.
Cook has made just one appearance since returning from the laceration. He threw 66 pitches for Pawtucket against Syracuse on Monday, allowing three runs on seven hits in four innings. Valentine said he expects Cook to slightly build upon that pitch count Sunday.
Tony Lee is a regular contributor to ESPNBoston.com.