Jose Mourinho believes Cristiano Ronaldo cannot leave Real Madrid, as the Champions League finalists would refuse to sanction the sale of their prized asset.
The Manchester United manager worked with Ronaldo at Madrid, who face Liverpool in Kiev on May 26 as they seek a third consecutive Champions League crown.
Ronaldo has regularly been linked with a return to Old Trafford and Mourinho conceded United would be interested in such a move - if it were possible.
But with Madrid seemingly determined to hold on to their prized asset, Mourinho expects the 33-year-old Portugal superstar to remain at the Santiago Bernabeu.
"I believe that Real Madrid cannot sell him, it is prohibited," Mourinho told Portuguese newspaper Record.
"He has to stay until the moment they authorise him to go and play one year in America or anywhere else that he wants to enjoy. It is a closed door."
Of a potential United return for Ronaldo, Mourinho added: "I don't think so, but all the people know his history in Manchester, the passion they have for him and that there are players in the world who, if the clubs have the possibility of signing, will not say no."
Mourinho indicated he had the chance to link up with Ronaldo by coaching the Portuguese national team, but he does not want to leave club management until later in his career.
"I almost went once, but [Real president] Florentino Perez didn't let me, because he thought that the position of Real coach was not compatible with the national team," Mourinho said.
"Only when I'm tired of the day-to-day, the three matches a week, the seven training sessions per week, and the one-month pre-season, things that don't tire me now."