The Cubs announced he will be placed on the DL with lat tightness and shoulder fatigue, per Bob Nightengale of USA Today.
Gordon Wittenmyer of the Chicago Sun-Times reported Thursday the Cubs were "bracing for the possibility" Lester could miss the rest of the regular season.
Maddon said after the team's 13-10 loss against the Reds that he was concerned but didn't want to draw any premature conclusion.
"When a pitcher of his stature is potentially injured, of course, you're concerned," he said, per Muskat. "I don't want to jump to conclusions either. Let's go through the proper procedures, let the doctor see him and go from there. Right now, I'm not going to jump to any negative conclusions."
Lester has had a rough season with a 4.37 ERA, but the four-time All-Star leads the Cubs with 26 starts and 148.1 innings pitched.