Mata is scheduled to undertake a United medical on Thursday as he puts the finishing touches to his club-record transfer to Old Trafford.
There was a breakthrough in talks between Chelsea and United on Wednesday, with the London club giving the 25-year-old permission to miss first-team training in the morning.
Barring any late hiccups, Mata is expected to make his debut for David Moyes’ team in their Premier League clash against Cardiff City next Tuesday.
Chelsea, who have had an approach for Wayne Rooney rejected in the January window, are set to make an immediate attempt to reshape their squad by trying to prise Salah from Swiss champions Basel.
The 21-year-old Egyptian made a series of eye-catching appearances in the Champions League before Christmas, scoring the equaliser for Basel in their unexpected 2-1 win at Chelsea in the group stage and repeating the feat in the 1-0 win at St Jakob Park.
The Londoners are prepared to reinvest around €12m, a quarter of the Mata money, in Salah although Basel value the wide player at closer to €18m.
Chelsea’s strong interest could scupper Liverpool’s bid to sign the Egyptian this month.
They will make their move after negotiating a big profit on Mata, who cost €28.7m when he signed from Valencia in 2011.
Mata told Chelsea he wanted to move to Old Trafford to play regular first-team football at a top club and boost his chances of playing for Spain at next summer’s World Cup. He is believed to have been offered a significant increase on his £70,000-a-week salary.
The Spaniard has been named Chelsea’s player of the year in the last two seasons but Jose Mourinho suggested earlier this month that the ‘door is open’ should Mata seek a transfer away from Stamford Bridge in the January transfer window.
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.
With United no longer considered a direct rival – the champions are currently seventh in the Premier League table, 14 points behind leaders Arsenal – Chelsea decided to sanction his sale.
It represents a major coup for Moyes, who has been given €180m to spend on new recruits by United in the two transfer windows covering this month and the summer.