Ronaldo joined Real Madrid in a world record £80m deal last summer.
“I don’t think Real Madrid is an easy club to play for – there is a circus attached to it,” said Ferguson.
“He knows the value of Manchester United, and Ruud van Nistelrooy has said the same because what they have here is protection.
“They come to training every day at Carrington and there is no one here but the players and the coaching staff.
“At Real Madrid and some of the big Italian clubs there are 2,000 people watching training.
“The media film the training sessions every day. It is completely private here at United and we don’t allow that exposure of our players.
“You’d like to think Ronaldo would want to come back here but you never know.
“I don’t think he will stay at Real Madrid all his life – there are other challenges for him.”
Fergie has admitted that he is still in touch with the Portuguese winger.
“I still have a great bond with Cristiano – he’s a good guy,” admitted the Scot.