The five-time Ballon d’Or winner has been linked with a return to Old Trafford in recent windows, with nine years having passed since he departed for Spain in a then record-breaking deal.
Ronaldo has never shied away from expressing his fondness for the Red Devils, with the 33-year-old having refused to rule out a second stint in England.
Mourinho, though, doubts that a deal can be done, with Madrid having tied their talismanic forward to a deal through to 2021 in an effort to keep potential suitors at bay.
"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."
Mourinho is prepared to admit that he would welcome the opportunity to sign Ronaldo, having previously worked with him at Santiago Bernabeu, but he is not expecting United to make a move this summer.
Quizzed on whether questions will be asked of Madrid, he 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.”
It would appear that Ronaldo and Mourinho are set to remain in their current roles for 2018-19, with the latter having committed to a contract extension at Old Trafford this season.
He is hoping to oversee more trophy triumphs at United, with an FA Cup final clash with Chelsea to come on Saturday, with the 55-year-old once again moving to play down talk of a possible move into international coaching at some point in the near future.
Asked if he intends to take charge of Portugal at some stage – a position he once considered combing with a role at Real Madrid – Mourinho said: "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.
"[I'll consider it] 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."