Lee came out of Saturday's game after an awkward swing aggravated his back. He had a similar problem in May 2007, and was sidelined for one week.
Maybe the time off will help. Last year on May 19, he hit a pinch-hit grand slam for an 11-6 Interleague win against the White Sox in his first at-bat after missing five games.
"There's really no news, I'm fine," Lee said Sunday. "Today's just a precautionary day. I just had some spasms [Saturday]. That's all it is. I feel fine, it's no big deal."
Lee was receiving treatment, and could be back in the lineup on Monday when the Cubs open a brief three-game road trip in Pittsburgh.
On Saturday, he struck out swinging to end the fifth and then hunched over in pain.
"Every once in a while, I get spasms in there," Lee said. "[Saturday], I took a real awkward swing and it gave me a little scare, but it's no big deal."
Lee has received treatment in the past for the spasms, but said it's not something he has to deal with on a daily basis.
"It's something that doesn't ever go away," he said. "It's not something that affects you daily. [Saturday], you saw the awkward swing and it flared up."
Daryle Ward started at first base Sunday in place of Lee, who was batting .290 with 18 homers and 74 RBIs. Lee has hit only three home runs since the All-Star break. Maybe the spasms have something to do with that?
"No," he said. "It was just a funny swing and it flared up."
Also missing from Sunday's Cubs lineup was outfielder Kosuke Fukudome, who was 1-for-13 on this homestand, and is hitless in his last four games. Fukudome looked at video of his swing and met with Cubs manager Lou Piniella before the game.
Carrie Muskat is a reporter for MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.