The 32-year-old spent six seasons at Old Trafford and has been linked to a return, but his compatriot says the move will not happen.
A return to Old Trafford was highlighted as a possibility after it was reported the 32-year-old has expressed his desire to leave Real Madrid while allegations of tax fraud are being pursued by the Spanish authorities.
The Portugal star won three Premier League titles and the Champions Leagueat United before his £80 million move to the Spanish capital, where he worked with Mourinho for three years.
Despite their history together, though, the coach says there is no way for his compatriot to return this summer.
“We have never thought about it because he is such an important player for his club, of great economic power,” Mourinho said after his side’s 5-2 friendly win over LA Galaxy on Saturday.
“We have not been able to find for whatever reason that could have made us think that Ronaldo could leave.
“I am not going to get my club to waste time on players who are mission impossible.”
Ronaldo spent six full seasons at Old Trafford, helping Sir Alex Ferguson’s Red Devils to nine trophies during that time.
The Portuguese was named Premier League Player of the Month on four occasions, Player of the Season twice and claimed the 2007-08 Premier League Golden Boot with 31 goals in 34 appearances.
In all, the four-time Ballon d’Or winner scored 84 and assisted 34 in his 196 top-flight appearances in England.
Ronaldo secured hero status at United as part of one of the most iconic teams in the club’s history, and has long been linked with a return to the club, despite signing a long-term deal in the Spanish capital.