The Manchester United boss has been linked with a sensational move to lure Ronaldo back to Old Trafford from Real Madrid.
Ronaldo is said to be unsettled in the Spanish capital due to his ongoing tax problems with the Spanish authorities.
United sold Ronaldo to the European champions for £80m in 2009, but Mourinho reckons a move to bring him back would be 'impossible'.
Starsport revealed last month how United executive vice chairman Ed Woodward had given up on singing the Portuguese superstar once and for all and Mourinho said: "I'm not going to waste my time thinking about players who are mission impossible.
"We have never thought about it because he is such an important player for his club, of great economic power.
"We have not been able to find, for whatever reason, something that could have made us think that Ronaldo could leave."
Mourinho, meanwhile, admits he is sad that United have been unable to agree a deal with Real for Spanish international Alvaro Morata.
United bosses pursued him for more than a month, but ended up refusing to meet Real's asking price of around £80m.
United decided to sign Romelu Lukaku instead and Mourinho added: "Clubs out price players, not the one who want to buy.
"He belongs to Madrid. Madrid has decided and we have not come to an agreement. It is simple.
"It is a shame. He is a player who made it to the first team with me. I have a special liking for him, he is someone special.
"I enjoyed watching his evolution in Juventus and Madrid. It is like that. I don't think he can come to us."
