Jose Mourinho's squad were given two days off this week, during which time the manager underwent surgery on a long-standing elbow complaint in Paris.
But he was back in charge at Chelsea's training base in Cobham, Surrey, on Wednesday as his fit players, with the exception of Mata, were put through their paces.
There are unconfirmed reports that a £37million deal is already in place and that the playmaker will have a medical at United before the weekend.
However, sources close to Mata have told Sky Sports News that this will be a difficult transfer to complete.
The 25-year-old is likely to cost United a club-record fee, exceeding the £30.8m they paid to sign Dimitar Berbatov in 2008, and the club are adamant they will not break the bank to sign him.
Early indications were that negotiations were going well, with United desperately in need of a fillip in a disappointing season but the issue of price could prove to be thorny.
Yet the desire for a deal to be agreed remains. Sky Sports' Spanish football expert Guillem Balague says all parties, including Mata, who has fallen out of favour at Stamford Bridge, want the transfer to go ahead.
Balague revealed on Revista de La Liga: "The idea is that Mata, who Chelsea are happy to let go, goes to Old Trafford in this transfer window.
"United have said no to the money - but I am under the impression that a deal will be reached because it benefits everybody.
"United need quality, Mata needs to get out of the situation he is in and it would reinforce United for their meetings with (Chelsea's title rivals) Manchester City and Arsenal, which is clever from Jose Mourinho."