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.
Phil McNultyChief football writer
"The club-record move for Juan Mata is not only a sign of ambition and support for manager David Moyes - it is a sign of the urgency with which improvement is required.
"United get a player who will add much-needed quality and creation to their out-of-sorts side.
"The intrigue surrounds where Moyes will play Mata if and when all his attacking resources such as Wayne Rooney and Robin van Persie are fit again. The Spain attacker's best position is just behind the strikers, so how will Moyes adjust his team to fit him in with the gifted Adnan Januzaj also available?"
The Portuguese has trusted Eden Hazard, Oscar and summer signing Willian in attacking roles this season and while his relationship with Mata is healthy, he has conceded Chelsea's system does not suit the player.
"There are fantastic players, but sometimes the fantastic players are more fantastic to a certain system and less fantastic in a certain model of play," Mourinho told BBC Sport's Football Focus.
Mourinho on Mata's future (Football Focus)
"He is an unbelievable player and a fantastic boy who is trying his best to adapt."
He followed it up with 20 goals in the 2012-13 season as Chelsea lifted the Europa League under Rafael Benitez but, since Mourinho returned for his second spell at the club, Mata has seen his playing time limited.
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