Bradley was hit by a pitch to lead off the fourth, and one out later, moved up when Milwaukee's Jeff Suppan walked Mike Fontenot. Ryan Theriot then singled, and Bradley pulled up as he jogged into third.
Cubs athletic trainer Mark O'Neal and manager Lou Piniella both talked to Bradley, who was then pulled from the game. The outfielder was batting .059 with one hit, a solo homer, in 17 at-bats.
Piniella wasn't sure how Bradley got hurt.
"We had extra batting practice today, and I don't know if that had anything to do with it with the coolness," Piniella said. "He said he felt it as he approached second base. He made a wise choice of just pulling up at third, because if there is something wrong, he could've caused a little damage. We'll see how he is and we'll have a better idea on Monday."
Reed Johnson entered the game as a pinch-runner for Bradley and ended up making a game-saving catch when he robbed Prince Fielder of a grand slam in the fifth.
"I knew that was the situation when I was watching on TV that Milt had a tough time," Johnson said. "That's a ball he normally scores on. I think if he had ended up going, he could've hurt himself more. I knew at that point I had to get loose in a hurry."
Johnson got an at-bat in the fourth as the Cubs sent 10 men to the plate. He'll most likely be in right field on Monday in place of Bradley.
Carrie Muskat is a reporter for MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.