The Cubs placed Thielbar on the 15-day injured list Friday with a left hamstring strain.
Thielbar had to come out of Thursday's game against the Phillies early after suffering an injury on the mound, and he'll now be forced to sit out at least another 15 days. The 39-year-old had been serving as Chicago's closer with Daniel Palencia (oblique) on the injured list, but that responsibility could be passed on to Ben Brown for the time being. Vince Velasquez was selected from Triple-A Iowa to fill the open spot in the Cubs' bullpen.