"Clay won't make his start tomorrow. He's not feeling well," said manager Bobby Valentine.
Cook was scheduled to start on Saturday for Triple-A Pawtucket in his latest rehab outing. The right-hander made a spot start for the Red Sox on May 5 vs. the Orioles, but he wound up with a left knee laceration when he was spiked while trying to cover home plate on a passed ball and exited the game after allowing seven runs over 2 2/3 innings.
With such a small sample size, the Red Sox aren't sure what to expect from Cook.
"We're playing that game of no expectation, let's see what he has when he gets here," Valentine said.
The club will announce a corresponding roster move on Sunday.
The Red Sox will likely skip Buchholz's turn in the rotation all together, rather than push the right-hander back a few days.
"We'll see how he feels. I doubt he's going to be pushed back, probably going to be skipped," Valentine said.
Buchholz was not at Fenway Park on Saturday. He's 8-2 with a 5.53 ERA this season and has won four straight starts in June.
Cook threw four innings in his first rehab start with the PawSox on Monday, allowing three runs and seven hits with one walk and three strikeouts vs. Syracuse.
"His sinker was going good, his secondary pitches we OK, too," Valentine said.
Cook will make his 11th career start vs. the Braves on Sunday. He is 1-2 with a 5.18 ERA in 12 appearances against Atlanta, all as a member of the Rockies.
Austin Laymance is an associate reporter for MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.