Grabow has been out since May 31 because of a left MCL sprain. He missed time earlier in May with the same knee problem.
Grabow pitched the eighth inning of the Cubs' 9-5 loss to Oakland on Tuesday. He allowed one hit, giving up a single to Rajai Davis.
The veteran pitcher, who signed a two-year contract with the Cubs in the offseason, has been ineffective most of the 2010 season. Grabow's role was to be the primary setup man for closer Carlos Marmol. That role has been taken over by Sean Marshall.
Grabow entered Tuesday 0-3 with a 9.45 ERA in 23 games this season. He made four rehab appearances with Iowa and allowed three earned runs in four innings.
Atkins was called up for Saturday's game against the Chicago White Sox. He did not make an appearance. He is 4-2 with one save and a 2.63 ERA in 14 appearances (six starts) with Iowa.
Bruce Levine covers baseball for ESPNChicago.com. Information from The Associated Press contributed to this report.