"It doesn't bother me swinging, just moving around on defense and trying to run," Lee said on Monday. "It gets pretty tight."
Lee has homered in three straight games for the sixth time in his career, and the first time since June 18-20 last season. However, he's hitting .251 overall. He didn't blame his back problems on his numbers this year, saying it's tough sometimes to bend over, but it doesn't affect his hitting.
Manager Lou Piniella had a little trouble explaining what was wrong prior to Monday's game.
"Derrek has a minor 'ball disk,'" Piniella said, then corrected himself. "Minor bulge disk. How about that? 'Ball disk.' I feel like a mechanic."
The problem is on the right side of Lee's lower back.
"He went to [athletic trainer Mark O'Neal] during the game [Sunday], and right away, we took him out of the ballgame," said Piniella, who removed Lee from the game as a precautionary measure.
Xavier Nady, who has hit safely in his last five starts, batting .316 with three doubles and four RBIs, started at first base on Monday in the Cubs' series opener against the Padres.