Ronaldo played for United between 2003 and 2009 in an illustrious part of his career where he scored 118 goals for the club in 292 games. He also won nine trophies with Sir Alex Ferguson.
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"I think it's difficult for that to happen, in truth," Mata told Goal in an exclusive interview.
"The respect and admiration that the people have for Cristiano (in Manchester) is incredible. You see it at the training ground, at the stadium, with so many photos of when he won all those titles there and did so well on a personal level as well.
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"But it's complicated with him at a club like Real Madrid and happy there, which I think he is. He is a player who makes the difference in any league, obviously. But as it's very complicated at the moment, I prefer to focus on the players who are here, on playing well and achieving the club's objectives."
Mata also commented on the upcoming El Clasico saying English football fans will also be glued to the encounter.
"A Clasico is always a game that is followed very much throughout the world and, obviously, in England too," he said. "English fans follow the Spanish league a lot. And a game like the Clasico is the game, with thousands of fans following it.
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"People in England always speak before, after and during a game. It's similar to what happens in games between Manchester United and Liverpool, or Manchester City, Chelsea and Arsenal... they are very big games.
"In the Premier League there are very big matches followed on a world level, because there are a lot of derbies"
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