Marmol, who went on the DL with a right hamstring strain on May 12, finished a rehab assignment at Iowa on Sunday with one scoreless inning, walking one batter while striking out three.
"I feel great and I'm ready to pitch," Marmol said. "I was able to find my arm slot again where I can get my slider back to breaking the right way."
Dolis had four saves in six attempts but recently struggled with his control, walking six batters over his last 1 2/3 innings and allowing six earned runs in a three-game period.
"We told (Dolis) it's not so much a demotion as it is to go down there and get it right," Cubs manager Dale Sveum said. "He has to be able to throw strikes. It is hard for me to use him right now because he has lost his fastball command. Hopefully he gains some confidence real quick and he won't be down there too long."
The bullpen has only six saves in 14 opportunities, although the Cubs' latest losing streak -- 12 games going into Monday's game against the San Diego Padres -- has had more to do with a lack of offense (33 runs scored in 12 games) than with pitching.