The Primera Liga champions have long been interested in the Portugal winger who has had a phenomenal season - with his 41 goals this campaign firing United to the Premier League title and the Champions League final.
Reports in Spain have linked Real with a £100million bid for Ronaldo but Calderon told Spanish TV programme ´Va de Futbol´: "It´s very difficult because he has a contract with a club that don´t want to sell him. It´s almost impossible, but not completely impossible because things can change between now and when the transfer window shutting."
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