McGee, who is 1-2 with a 2.57 ERA and six saves in 36 games this season and 21-11 with a 2.80 ERA over his six-year career, felt something give in his knee toward the end of his outing Tuesday against the Astros.
The timetable for McGee's return is 6-8 weeks, potentially putting him back on the mound sometime in early October. McGee was placed on the 15-day disabled list retroactive to Wednesday, and the tear was diagnosed Thursday.
"He's got a ways to go, but get the rehab and we'll see how he does when he starts throwing in a couple weeks or so," Cash said.
Trevor Hass is an associate reporter for MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.