The Spaniard was due to fly from London to Manchester to complete his £37m move yesterday but the Blues' negotiations for Mohamed Salah halted the deal.
The Spaniard is believed to have agreed a four-and-a-half-year deal at £150,000 per week, but is waiting on the Blues to formally sanction his departure before he completes the switch.
As late as last night, Chelsea were reportedly refusing to countenance Mata's departure as they acted swiftly to conclude negotiations with Swiss side FC Basel for the signing of Mohamed Salah.
A team of United staff had arrived at the club's Carrington training complex to lay out a helipad ahead of Mata's scheduled arrival on a helicopter from London to Manchester but the flight failed to materialise.
United were therefore unable to proceed with the medical examination but there are no fears on the club's behalf of the deal failing to go through, with the club-record move expected to be finally sealed by the end of the weekend at the very latest.
Mata has become disillusioned with life at Chelsea under Jose Mourinho and has appeared at odds with the Portuguese's philosophy since the manager returned to Stamford Bridge for a second spell in charge.
United were readying a bid on Monday and talks have come to a speedy conclusion which will see the Blues' two-time Player of the Year join the struggling reigning champions, who were dumped out of the Capital One Cup by Sunderland in midweek and face a fight to finish in the top four.
Will Mata bring about a change in fortunes for Manchester United?
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