Sir Alex Ferguson has said he would like to see Cristiano Ronaldo one day return to Manchester United though he admitted it was a "long way off".

Ronaldo spent six years at United before his world record £80m transfer in 2009 to Real Madrid, the club United face in the last 16 of the Champions League. Asked if Ronaldo might come back and play for United, Ferguson said: "I don't see that. 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 second, I don't think there's any chance they would want to sell him.

"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."

Ronaldo's affection for United, where he won the Champions League, three Premier League titles, the FA Cup and two League Cups, was underlined when he told the Spanish newspaper AS he would not go overboard if he found the back of the net in the Champions League. "I won't celebrate if I score against United," he said. "I still speak with Ferguson. I miss him a lot and I miss Manchester United. I felt very good there; it was like a family. I had six great years there and am still friends with my team-mates. It is thanks to Manchester United that I am here playing for Real Madrid. Without them I wouldn't be the player I am today."

Ferguson said Ronaldo's courage is one of the attributes that place him among football's elite. "Great players have got that. All the great players you look at, particularly forwards have got that – Denis [Law], [George] Best – they always take the ball no matter the challenges. Pelé, Johan Cruyff, Eusébio, all the great players can do that. They take the ball, no matter the challenges," the United manager said.

"There are different forms of courage and that one is the best one. You've got lads who will run through a brick wall, lads who go into flying tackles, they will play with cut heads, but the courage to take the ball all the time is a fantastic courage. He always had that."

Asked if Ronaldo is the best Ferguson has managed, he said: "I've been very fortunate in having Scholes and Giggs, I've had fantastic players – Cantona. He's certainly in that bracket, there's no question about that."

Ferguson joked that it will be difficult to stop Ronaldo. "The contract with Real says that he can't play against us – I think it will be a disgrace if he plays against us anyway. He has no loyalty to us! One thing is for sure, that he'll get a great reception when he comes to Old Trafford and quite rightly when he comes on the pitch. After that they can boo all they like."

Real's manager, José Mourinho, first gained major attention with his touchline celebration at Old Trafford when Porto beat United on the way to winning the European Cup in 2004.

Ferguson said: "I've not got a great record against José and I need to put that right. The main focus will centre around Cristiano. It will be a fantastic tie and hopefully we can get them back here with a good chance and the place will be buzzing. We've 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."