Dempster pitched seven shutout innings on Saturday against the Los Angeles Dodgers despite the blister, which has apparently been bothering him all season.
If the Cubs' rotation stayed in alignment, Dempster's next game would be Thursday in Atlanta. But because Carlos Zambrano is eligible to come off the suspended list at that time, he'll start on Thursday and Dempster can go either Friday or Saturday in Cincinnati.
"If he needs an extra day, we'll give it to him," Cubs manager Lou Piniella said of Dempster, who is 4-3 with a 4.48 ERA.
Rich Harden, on the disabled list with a mid back strain, will throw off the mound on Tuesday in Atlanta, and the Cubs will then determine the next step in his rehab. Harden, who has not pitched since May 18, said he had no problems after a long-toss session on Sunday.
Piniella did indicate Harden (4-2, 4.74 ERA) could return during the upcoming nine-game road trip, but on Sunday was more cautious.
"Let's wait until [he throws in] Atlanta, and then we'll have a better idea for you," Piniella said.
Alfonso Soriano has been bothered by a sore left knee, but says it's not bad enough to make him miss a game.
"He likes to play," Piniella said of Soriano, who was batting .122 (5-for-41) in his last 10 games after hitting .290 from May 2-19. "I've talked to the trainer about his knee, and it doesn't prohibit him from playing."
Soriano feels some discomfort when he lifts his leg as part of his stride at the plate. But he didn't want a day off.
"Every day, it's getting better and better," he said on Sunday. "It bothers me to play but not to take one or two days. I don't remember my last game when I played 100 percent and nothing was bothering me."
He was ranked among the National League leaders in home runs but has not hit a home run since May 17.
"I have to be more selective at home plate," Soriano said. "I'm having a little slump, but I think it's because I'm swinging at bad pitches."
Infielder Ryan Freel, on the DL with a strained left hamstring, was to fly to Arizona on Monday and work out at the Cubs' facility there.
Carrie Muskat is a reporter for MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.