Right-hander Rafael Dolis was optioned to Triple-A Iowa.
Marmol, who was placed on the DL on May 12 with a right hamstring strain, made two rehab appearances for Iowa. He allowed one hit in two innings and, on Sunday, struck out the side while walking one and throwing two wild pitches.
"I feel very good, very comfortable -- that's more important," said Marmol, who is 0-1 with a 6.35 ERA in 15 relief appearances for the Cubs this season.
Marmol, who has 16 walks in 11 1/3 innings, said he worked on his fastball and slider and feels comfortable with both. Manager Dale Sveum said Marmol would pitch in the seventh inning and as matchups suggest, the same role he was in prior to his injury.
Now it's Dolis' turn to figure out things in Triple-A. Dolis was 0-2 with a 24.00 ERA in his last five outings.
"It's not a demotion as much as just going down there and getting right and getting back here, hopefully, pretty soon," Sveum said. "He's got to be able to throw strikes. It's hard for me to use him right now because he's lost complete command of his fastball. Go down, hopefully gain some confidence real quick, and he won't be down too long."