Bradley, who was batting .196 this month after hitting .311 in August, was lifted from Thursday's 7-4 loss to Milwaukee after he singled in the sixth.
"I don't know [how long]," Cubs manager Lou Piniella said. "If he feels better tomorrow, we'll get him in there."
Bradley's exit Thursday looked unusual because he singled, ran to first, then headed for the Cubs' dugout. No one knew what was going on except those in the Cubs' dugout, because he had told Piniella and athletic trainer Mark O'Neal before the at-bat that his knee was sore.
After the game, Bradley did not want to answer questions about his knee. Piniella heard about the terse responses.
"I think you just answer the question," Piniella said. "You get asked the question, you answer it and you're done with it. It's simple. If a guy can't run well in a one-run ballgame, it's my job as the manager to get somebody else in there, and that's exactly what we did. Answer the question and go on to the next one."
How hard is that?
"It shouldn't be all that hard," Piniella said.
Micah Hoffpauir started in right field in the first game of the Cubs' series against the division-leading St. Louis Cardinals. The Cubs don't have many outfielders to call upon with Alfonso Soriano, Reed Johnson and Sam Fuld also sidelined with injuries.
Reliever Angel Guzman tried to throw before Friday's game but is still bothered by a sore right triceps, so he isn't available either.
"We are banged up," Piniella said. "We're banged up a little in the bullpen, banged up in the rotation and banged up in the lineup.
"You want to go out there and win every game you play and as many games as you can, but you have to be realistic, too. [This road trip] isn't going to be easy."
Carrie Muskat is a reporter for MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.