The four-time Ballon d'Or winner was reportedly considering his future at the Bernabeu earlier this summer after being left upset by allegations of committing tax fraud.
Eight years after leaving Old Trafford to join Los Blancos in a then-record deal, Ronaldo was tipped to rejoin the Red Devils, where he would link up with his former Madrid manager.
Mourinho does not believe that United have any real chance of tempting his Portuguese compatriot to leave Madrid, however, claiming that he would simply be wasting his time in trying to do so.
"We have never thought about it because he is such an important player for his club, of great economic power," he told reporters. "We have not been able to find for whatever reason that could have made us think that Ronaldo could leave.
"I am not a defender in my club once you waste time on players who are mission impossible."
Ronaldo, who spent six years with United earlier in his career, is currently enjoying an extended holiday after playing a part in the recent Confederations Cup.