Strop injured himself while working out several days ago but tried to pitch through the pain. He left Tuesday night's game because of the injury after just 1/3 inning. He gave up three runs on three hits to the Chicago White Sox in a 5-1 loss.
Manager Rick Renteria was asked if it was a good idea that Strop pitch while not 100 percent.
"I didn't think it was an issue, quite frankly," Renteria said. "He had been working. All indications were he was fine."
The Cubs recalled left Zac Rosscup from Triple-A Iowa to take Strop's place. It's Rosscup's third time with the Cubs this year. He's thrown 2 2/3 scoreless innings.