The Manchester United manager has waxed lyrical about his former star and would like to think a return is possible but admits that he does not think it will happen
The Portugal captain's disquiet at Madrid was well-broadcast earlier in the season and he has recently declared his admiration for Sir Alex.
Ronaldo moved from the Manchester side to Real Madrid in the summer of 2009 for £80 million and has since scored an incredible 165 goals in 162 games for Los Blancos.
Sir Alex acknowledges that a return would be ideal but, given the amount of money involved, remains highly unlikely.
"I'd like to think it would be [possible], but I don't think it will be. You're talking about incredible amounts of money now," he told reporters.
"What would you think his value would be? He's definitely unbuyable. Who could afford to buy him?"
He added: "He was definitely good value for Madrid, but he always wanted to play for them. It was his boyhood dream. I’m not saying he'll stay there forever, you never know... How do you know we've not got a buy-back clause for him?"
The Red Devils boss proceeded to express his appreciation of his former protege, comparing him with the very best that United have had.
"He's an unbelievable player - one of the best. We have a pantheon of great players and Ronaldo definitely compares with any of them," he continued.
"Although he had a natural talent, he is in many ways manufactured himself. He practised and practised. He had a bit of the Eric Cantona about him in that way. Paul Scholes is still the same."