The Portuguese international spent six years at Old Trafford before moving to the Spanish capital for a then world-record fee.
“You never know," he told talkSPORT. “I know when I signed him, he was saying that Manchester for him has been like his house, he was quite happy there with the fans, with the club.
“At the time with [Sir Alex] Ferguson, he considered him like a father, and everything for him in Manchester was perfect.
“But he wanted to come to Madrid and play for Real, and you never know, maybe at the end of his career he would like to come back to England and back to Manchester.
“His strength is that every day he gets up in the morning and thinks only about being better than the day before.
"This makes him unbeatable. I haven't seen a player like him in my life. I don't see the limit - I don't know what is going to be the highest level for this player.”
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