The Spanish star has agreed a four-and-a-half-year contract worth £150,000-a-week at Old Trafford, but Chelsea are yet to complete their side of the deal as they want to make sure they have a replacement tied up first.
Mata was due to arrive in Manchester on Thursday to undergo a medical but that had to be pushed back 24 hours as the London club refused to sanction the move as they prepared to sign FC Basel's Mohammed Salah.
Ground staff at United's Aon Training Complex at Carrington had arrived early on Thursday morning to prepare a helipad in order for the Spain international to arrive by chopper in the manner of a superstar signing.
Despite the delay, United remain confident that a deal will be completed for Mata within the next 48 hours and senior figures at the club are currently unconcerned by the hold up at the Chelsea end.
Mata cannot wait to get started and, with the move to go through on Friday or over the weekend, the 25-year-old midfielder looks set to be unveiled on Monday in time to make his debut at home to Cardiff City on Tuesday.
Mata wore the number 10 shirt for Valencia and Chelsea but could be offered United's iconic number seven, given up by Antonio Valencia at the start of the season.