He is scheduled to pitch again on Sunday and could rejoin the big league team on Monday or Tuesday.
When he does return, he will not automatically be inserted into the closer's role, which was his job at the start of the season but has since been given to rookie Rafael Dolis.
"I think he'll still have to prove to me that he can throw strikes," manager Dale Sveum said. "It'll be the same game plan as when he was taken out of the closer's role. Nothing has changed that way."
In other injury news, catcher Steve Clevenger, who has been sidelined since April 26 with a strained right oblique, was to play nine innings for Iowa on Monday and again on Wednesday. He would then be re-evaluated to see if he is ready to rejoin the Cubs.
Reliever Blake Parker, who has been hampered by a tight right hamstring and not pitched since last Saturday, was available to pitch on Friday against the Pirates.