Cardinals manager Mike Matheny said Lackey has been dealing with stiffness and a dead arm.
"He's not really excited about this, and I get that, too," Matheny said. "The guy wants the ball. We're in September and we're pushing down to the end here and he should want to be in there, every single one of those guys should, and we get that. We just have to figure out what gives us the best chance, and sometimes it's helping a guy take a rest when he doesn't even want it."
Barring any setbacks, Lackey is expected to pitch against Cincinnati in the final series of the homestand.
"Starting pitchers, they get beat down," Matheny said. "That's just part of it. Not the first time, I'm sure, for him, either. He's been through this before. You've got to push through it when you can. You've got to take a little break when you can."
The move gives the Cardinals their three hottest starters for the Milwaukee series. The Brewers entered Sunday five games behind St. Louis in the National League Central.
Joe Harris is a contributor to MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.