Ronaldo, 27, spent six years in the Premier League under Ferguson’s tutelage and will return to his former club for the first time since 2009 when Real take on United in the Champions League round of 16.
While Patrice Evra has called for Ronaldo to return to England, Ferguson is less optimistic about having the Portugal international back in his side.
“I don’t see it,” Ferguson said. “I would love to think it. Maybe when he is about 39 and you are coming past my grave, but it is fanciful thinking really.
“First of all, how much would it take to get him from Real Madrid? Secondly, I don’t think there is any chance they would want to sell him anyway.
“You never know. I don’t know when his contract finishes and he may want to go to another club at some point in his career. Of course I would hope he’d want to come here. But that is a long way off.”
Ronaldo scored six goals in the Champions League group stage and could stand in the way of United’s progression to the quarter finals this season, leaving Ferguson hoping that there could be a clause in the transfer deal with Real.
“Our contract with Real is that he can’t play against us. I think it would be a disgrace if he played against us. It would be no loyalty,” he laughed. “He will get a great reception when he comes onto the pitch at Old Trafford, quite rightly. After that they can boo him as much as they like.
“We are proud of the part we played in his career. We did very well to get six years out of a boy when he came to us at the age of 17.
“He always had a hankering to go to Real Madrid at some point in his career. It wasn’t disloyalty. I got another year out of him when he wanted to go the previous year.
“He honoured that and did fantastic for us and then went with our blessing. My way of looking at it was that we were lucky to have him for six years. The United fans would share that.”
In a match with numerous sub-plots, Ferguson will be pitted against his old foe Jose Mourinho and while the Scot is hoping to get the better of his rival, he believes the presence of Ronaldo will render the managerial battle insignificant.
“I don’t have a great record against Jose so I will have to put that right,” he said. “We have not had a really good European game for quite a while and this is a big opportunity to step forward in European football again.
“But the main focus will certainly centre around Cristiano. He has courage, like all the great players. Not in the sense of running into a brick wall but in the way they take the ball no matter the challenges. That is the best form of courage.”