On Tuesday, Chelsea insisted that Mata, who had been voted the club's player of the year two seasons in a row, was not for sale.
But the player, who was part of the Spain squad which won the World Cup in 2010 and European Championship in 2012, has been substituted in nine of his 13 Premier League appearances for the Blues this season, prompting speculation other clubs could bid for him in the January transfer window.
For Jose Mourinho, it is the culmination of a strategy organised by him to sell on the club’s player of the year for the past two seasons to another club that he believes to be out of the title race and who will play Chelsea’s rivals.
Should fate draw the two clubs against each other in the Champions League, the only other tournament they can meet in this season, Mata will be cup-tied.