Calderon: Ronaldo could return to United
The Portugal forward has cemented his place as one of the world's best players since leaving Old Trafford for the Bernabeu in 2009, winning the FIFA Ballon d'Or in January this year.
Ronaldo has netted 41 goals in all competitions this season and Calderon refused to rule out the possibility of him returning to Old Trafford at some stage.
"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 (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."
Real Madrid sit three points clear of local rivals Atletico Madrid at the top of La Liga and meet Barcelona - a point further back in third - in El Clasico on Sunday.
"It's clear now that this year, mainly the last two or three months, Real Madrid has been stronger than Barcelona," Calderon added. "They've played much better.
"Four points ahead is a good distance. In fact, if Real Madrid wins, it's going to be a problem for Barcelona to win this league.
"But don't forget Atletico, they are still there, they are playing well and it is a very strong team.
"I think in this match, Real Madrid has a slight advantage. Barcelona's midfield is weaker this year. I don't know why - maybe the players are in a different way of playing, maybe the change of the coach. I don't know."