McCann strained his left oblique during Tuesday night's 4-3 victory in 19 innings over Pittsburgh. He grabbed his left side and lower back in obvious pain after he threw high as Neil Walker stole second base in the 10th inning.
McCann is hitting .306 with 18 homers and 55 RBIs in 91 games.
The 33-year-old Nieves signed a $775,000, one-year deal with Milwaukee in the offseason. He made the opening-day roster, but struggled badly in the backup role. He hit .140 in 20 games before being sent to Triple-A Nashville, where he was hitting .170 in 23 games.