Strop left Tuesday night's 5-1 loss to the White Sox after recording just one out in the ninth inning. The right-hander was charged with three runs and three hits.
Strop had an MRI on Wednesday. He is 0-2 with a 4.97 ERA in 15 appearances this season.
The Cubs also recalled left-hander Zac Rosscup from Triple-A Iowa. It is his third stint of the season with the major league club.
The 25-year-old Rosscup pitched 1 2-3 scoreless innings at the New York Yankees on April 16, and threw another scoreless inning on April 29 in Cincinnati. He was sent back to Iowa on May 3.