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.Jose Mourinho Cristiano Ronaldo Real Madrid
"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."