The team recalled righty Dillon Maples to take his spot, but Cishek is the third key reliever on the team to go on the IL in the past week.
He joins closer Craig Kimbrel (right knee) and set-up man Brandon Kintzler (right pectoral) on the sideline as the team attempts to navigate a 10-game road trip. The first-place Cubs are 22-34 away from Wrigley Field, with a 4.73 ERA, fifth-worst in the NL.
"[The] opportunity is there for some people," Cubs manager Joe Maddon said. "Now is the time to take advantage of it."
The Cubs have just two relievers on their active roster who were there on Opening Day: Pedro Strop and Tyler Chatwood. Strop has struggled recently, so Maddon indicated that it could be any of his relievers pitching the ninth inning.
The most promising call-up from Triple-A has been righty Rowan Wick. He has pitched in 21 straight appearances between the minors and majors without giving up a run -- the final eight of those with the Cubs.
"You'll see Wick in a starring role tonight [Saturday], possibly," Maddon said. "Maybe co-starring [Duane] Underwood."
Underwood's first outing this season consisted of him facing six batters and striking them all out. The Cubs think they have enough to get by until their walking wounded return, but Maddon wouldn't name one person to take the ninth inning while missing so many veterans.
"They're high-end potential relief pitchers," Maddon said.
With a 2½-game lead in the division heading into their game against the Cincinnati Reds on Saturday, the Cubs think they can get their top pitchers healthy while giving their arms a break before the stretch run.
"That can be the benefit," Maddon said. "They get this week to 10 days of rest."
Kintzler threw a bullpen session on Saturday and is eligible to come off the injured list on Friday. Kimbrel will rejoin the team on Tuesday in Philadelphia and shouldn't be far from returning. Lefty Kyle Ryan should also rejoin the team next week, as he was placed on the bereavement list on Friday.