The United winger said: "After what he has done this season, Ronnie will be voted European Footballer of the Year and World Player of the Year - I'm certain of that.
"And if he carries on playing the way he is, he could be the best in the world for a very long time.
"The key is whether you can do it over a sustained period, and I'm confident Ronnie can do that.
"But we're recognised as the biggest club in the world and in Ronnie we've got the best player in the world. And the Champions League is the best stage there is to prove that.
"So why would Ronnie want to go anywhere else? He's already at the best club he could possibly be, and playing football on a different level to any other player in the world."