The All Star captain injured his ankle and knee in a club game at the start of May.
McManus has an appointment with a specialist this week to determine whether or not he will require surgery.
If the news is kind to the forward it would be the latter part of the championship before he could make a return.
The Farney County play Tyrone in the provincial semi final on Sunday week the 15th June in Clones but that game would come too early for him.
If surgery is needed McManus would be out for up to ten weeks which would effectively rule him out for the summer.