Grabow, who had been sidelined since May 31 because of a left knee sprain, joined the Cubs as they begin an Interleague series against the Oakland Athletics on Tuesday.
"Oakland has a lot of left-hand hitting, so he's back at the right time," Cubs manager Lou Piniella said of the Athletics, who are batting .250 against lefties, compared to .267 against right-handers.
Mitch Atkins, called up Friday from Triple-A Iowa, was sent back to the Minor League team. He did not get in any games during his brief callup and was expected to start for Iowa on Wednesday.
Grabow was 0-3 with a 9.45 ERA in 23 games with the Cubs before going on the disabled list.
"It was definitely bothering me," Grabow said of his knee. "I'm not going to make an excuse that that's why I was pitching bad, but it was in the back of my mind when I was throwing."
In four rehab appearances with Iowa, Grabow gave up three earned runs in four innings. He did not give up a run in three of his four appearances. He was roughed up Saturday, serving up three runs on four hits in one inning against Memphis. In the other three outings, he gave up two hits and one walk.
"I threw pretty good," Grabow said. "I pitched in four games, and had one bad one and the rest were real good. I'm happy to be back, and it's time to move forward now and help this team win."
With Grabow's return, the Cubs' bullpen now has three left-handers in Grabow, Sean Marshall, and Tom Gorzelanny.