The Red Devils agreed a 37 million pound fee with Chelsea for the World Cup and European Championship winner on Wednesday night.
The move is set to be finalized after the completion of a medical, which will take place at the new £10 million medical facility inside Carrington's grounds, with doctors already on site.
There was a breakthrough in talks between the two clubs on Wednesday, with the London club giving the 25-year-old permission to miss first-team training in the morning.
Mata is expected to make his debut for David Moyes’ side in their Premier League clash against Ole Gunnar Solskjaer's Cardiff City next Tuesday.
Chelsea will move for Basel's Mohamed Salah, who had been linked with Liverpool, as a replacement for the 25-year-old playmaker.
Details of the transfer emerged as United crashed out of the Capital One Cup to Sunderland on Wednesday evening.
Moyes' men missed four of their five shootout penalties after forcing extra time thanks to a last-gasp Chicharito goal, which was scored just seconds after Phil Bardsley looked to have settled the tie for the Black Cats.
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