Cahill sustained the injury June 19 against the Miami Marlins when he was struck by a line drive.
Cahill, 25, is 3-10 this season with a 4.66 ERA, 1.41 WHIP and 1.77 K/BB ratio. He also leads the majors with 11 wild pitches.
Cahill finished June with an 0-5 record and a 9.85 ERA in six starts.
"Just at this point, it's just getting kind of old," Cahill said after yesterday's 6-2 loss to the Atlanta Braves. "I feel pretty good going into the start and then I can't make pitches, I'm giving up runs and making the bullpen wear it every time. It's kind of embarrassing."
The team recalled right-hander Chaz Roe from Triple-A Reno take Cahill's spot on the active roster.SI WIRE: Nationals activate Harper from DL