"It's something he's been dealing with over the last couple of days," manager Jim Tracy said.
The big concern for the Rockies was not to do anything that would impact Buchholz's continued recovery from Tommy John surgery, which he underwent after the 2008 season.
"The program he is on is pushing on," Tracy said, referring to the exercise program that is part of his recovery. "When you are talking about a ligament transplant, that combination with lower back stiffness for me doesn't work."
Buchholz, while disappointed, understands the club's concern.
"They don't want me to put any extra stress on my arm by overcompensating for the back," he said.
The lower back problem is something Buchholz says he's had for a while.
"I've had some problems with the back spasming up before and I just have to fight through it," he said. "They didn't want to hold off for a couple of days, and then it cripples the bullpen. It's just bad timing."
The plan now calls for Buchholz to take the day off and then see how he feels Wednesday to see when he'll begin to work out again. Once they can get Buchholz's back to where it needs to be, he'll get some throwing in and eventually some Minor League games.
"Knowing that we have a couple of weeks period here," Tracy said, "and have him part of the scenario here in September when rosters expand."
"We'll see how it feels tomorrow," Buchholz said. "It's just bad timing."