The two European heavyweights will go head-to-head in the last 16 of the Champions League, and Real superstar Ronaldo will be facing United for the first time since he left the club in 2009.
Ferguson signed the Portuguese winger as a teenager, and in his six years with the club he became a global superstar.
Ferguson said, with tongue in cheek: "It is in his contract he can't play against us. It would be a disgrace if he played."
The Scot added: "It will be fantastic to have him back. When he comes on to the pitch at Old Trafford he will get a great reception, quite rightly - after that they can boo him as much as they like.
"You have to remember, he came here at 17 and developed himself into a great footballer. We are proud of the part we played in his career.
"To get six years out of a boy when he comes to us at 17, we have done very well."
Ferguson insists there is no ill-feeling about the manner in which Ronaldo left Old Trafford.
"He always had a hankering to go to Real Madrid at some point in his career and we helped him on that way," said Ferguson.
"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."
Ferguson says his personal battle with Real coach Jose Mourinho will be secondary to Ronaldo's return.
"I don't have a great record against Jose so I will have to put that right," said Ferguson.
"But the main focus will certainly centre around Cristiano. It will be a fantastic tie. Hopefully we can get them back here with a good chance and get the players absolutely buzzing.
"We have not had a really good European game for quite a while having not qualified last year.
"This is a big opportunity for United to kick on and step forward in European football again."