Speculation has been rife today that the Red Devils are closing in in on a swoop for the Spanish star, who has found first team football hard to come by following the arrival of manager Jose Mourinho in the summer.
According to reports this morning, Manchester United had held talks with Chelsea over the former Valencia man, and were told that a bid in the region of £37 million would be required to make the deal happen.
Now BBC Sport report that a formal offer in the region of £35 million has been made - although it has been made through intermediaries with Manchester United denying that they have made the bid themselves.
BBC Sport claim that Chelsea are likely to reject the initial bid and hold out for a fee closer to £40 million - but that a deal is certainly on the cards.
Mata reportedly didn't appear at training today as rumours of a switch to Old Trafford continue to gather pace - although Chelsea insist that this isn't the case.
The Spaniard, twice Chelsea's player of the season, joined the club from Valencia in a deal worth £23.5 million in 2011, but has lost his place in the starting XI to Brazilian Oscar.
Recently, Jose Mourinho insisted that Mata wouldn't be sold.
Manchester United are looking to spend big in the January window in an attempt to save their season. Under new boss David Moyes they sit six points off the top four race, with their hopes of defending their league crown long gone.