Marshall has held opponents scoreless in 17 of his 21 appearances and batters are hitting just .162 off him. Grabow has pitched better since receiving a cortisone shot in his left knee on May 8.
"We're basically almost strictly going left-handed in the seventh and eighth innings," Cubs manager Lou Piniella said. "We tried going the other way and it's worked much better with the lefties, so that's what we're going to do.
"I look at Marshall now as our eighth-inning guy, and I look at [Carlos] Marmol obviously as our closer -- and the seventh, we need to do what we need to do. You get them out left-handed, right-handed, it doesn't matter as long as you get them out."
Cubs general manager Jim Hendry has been looking for an experienced right-handed setup pitcher since the offseason.
"I don't worry about right or left," Hendry said Tuesday. "Grabow, for most of his career, has gotten righties out more than lefties. Marshall's been terrific. People aren't giving him enough credit. We label people in this role and that role.
"We'd certainly like to have another reliever who can pitch," Hendry said. "[Esmailin] Caridad will get more chances."
Zambrano was switched to the bullpen because the Cubs needed a more experienced setup pitcher. But he now will be stretched out and eventually be inserted back into the rotation.