A sensational return to Old Trafford could be on the cards for Cristiano Ronaldo after Manchester United boss Sir Alex Ferguson suggested he may be able to resign the striker in the future.

GettyImagesCristiano Ronaldo stands dejected after Real Madrid's Champions League exit to Lyon

The Portuguese star joined Real Madrid last summer in a world record £80 million deal.

Despite Real's solid league form they were dumped out of the champions League by Lyon this week, the sixth time in a row they have failed to make the quarter finals.

And Ferguson has hinted that Ronaldo may be struggling to adapt to life in the intense spotlight.

"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."

The United boss says he is still in touch with Ronaldo. "I still have a great bond with Cristiano - he's a good guy,'' he added.