Mata was absent from the Blues first-team training session in the morning as the clubs hold talks over a 40 million pound deal for the Spain international.
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Mata is desperate for regular football having found himself out of favor at Chelsea this season, starting just 11 of Chelsea’s 22 Premier League matches.
Chelsea initially demanded £45 million to sell Mata to a domestic rival but the club is prepared to accept an offer worth around £40 million for a player who cost £23.5 million when he signed from Valencia in 2011.
A deal for Mata could also improve relations between the clubs as Chelsea continues with its attempts to sign United striker Wayne Rooney.
Mata is keen on a move to Old Trafford to boost his chances of playing for Spain at next summer’s World Cup, and the Premier League champion can offer a significant increase on his £70,000-a-week salary.
He has been named Chelsea’s player of the year in the last two seasons but Mourinho suggested earlier this month that the ‘door is open’ should Mata seek a transfer away from Stamford Bridge in the January transfer window.
Mata is under contract until 2016 and Chelsea would prefer to evaluate his situation at the end of the season and sell him abroad, with Paris Saint-Germain and Atletico Madrid among the player’s admirers.
But with United no longer considered a direct rival - United is currently seventh in the Premier League table, 14 points behind league leader Arsenal - Chelsea is prepared to listen to big offers for the attacking midfielder this month.
It would represent a major coup for Moyes, who is under significant pressure and has yet to bolster his fragile squad in the winter window, despite having around £150 million available to spend on new recruits.
Mourinho has cited Mata’s lack of defensive work as the main reason behind the Spain international’s frequent omission, with Oscar preferred in the No.10 role at Stamford Bridge.
Mata has been Chelsea’s player of the year in the last two seasons, and last term scored 12 goals and provided 12 assists in the Premier League alone.