Zambrano followed starter Jeff Gray, and retired the side in the Cubs' 2-1 loss to the Royals.
Zambrano, who has been on the restricted list following his tirade in the dugout at U.S. Cellular Field on June 25, is expected to pitch for Iowa this weekend in Round Rock and join the Cubs next week during their trip to Houston and Colorado.
He will likely pitch for Triple-A Iowa either Wednesday or Thursday. Cubs roving Minor League pitching instructor Mark Riggins is monitoring Zambrano's workouts.
"Z's basically been feeling good, so we'll see," Cubs manager Lou Piniella said Monday.
The Cubs were expected to make a roster move and add a fresh arm prior to Tuesday's game after having to use three relievers in an 11-5 loss to the Astros.
Brian Schlitter, on the disabled list with right shoulder impingement since July 3, made his first rehab outing Saturday for Triple-A Iowa, throwing one inning. He was expected to throw Monday or Tuesday, then take a day off and pitch two innings. Piniella liked the right-hander's arm during his brief stay with the big league team. Schlitter appeared in only three games for the Cubs from June 28-July 2.
John Grabow, on the disabled list since June 29 with a sprained left knee, was also expected to start throwing and could be back by the end of July. The left-hander is wearing a knee brace during batting practice and workouts.