The Portugal captain left Old Trafford for Real Madrid in a £80 million deal in 2009, but has been linked with a move away from the Liga champions recently, with Paris Saint-Germain touted as a possible destination.
However, the 70-year-old admits he is not seriously considering a move for the attacker at the moment, and joked he may only be able to make an offer when his contract at the Bernabeu expires.
Sir Alex told reporters: “I’d love to see that, but it’s fanciful thinking really.
“First of all, how much it would take to get him from Real Madrid, and secondly, I don’t think there’s any chance they would want to sell him anyway. But you never know. I don’t know when his contract finishes.
“He may want to go to another club at some point in his career and I’d hope he would want to come here. But that’s a long way off.
“I’ve been very fortunate in having [Paul] Scholes and [Ryan] Giggs. I’ve had fantastic players – [Eric] Cantona. He’s certainly in that bracket as one of the best I have managed, there’s no question about that.”
Ronaldo will return to Old Trafford in the New Year as Madrid and United square off in the last 16 of the Champions League and Sir Alex is expecting a warm welcome back for the Portuguese.
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He added: “One thing is for sure, he’ll get a great reception when he comes to Old Trafford and when he comes on the pitch. After that, they can boo all they like!
“United fans have always been that way with players who performed for us. Players who are part of the history of the success of the club are recognised when they come back.
“We’re proud of the part we played in his career. To get six years out of a boy when he comes at 17 and played for us for six years, we’ve done well.
“He always had a hankering to go to Madrid in his career and we helped him on that way.
“It wasn’t disloyalty because I got another year out of him when he wanted to go the previous year. He honoured that and was fantastic for us. He went with our blessing.
“My way of looking at it is that we were lucky to have him for six years and the United fans would share that.”