Rumours are rife that the Red Devils could be on the verge of finalising a big-money deal for the Spaniard, who has fallen out of favour since the arrival of Jose Mourinho at Stamford Bridge this season.
Mata, twice Chelsea's player of the season, has made just 11 starts in the Premier League - and hasn't featured since his strop at being substituted during Chelsea's 3-0 win over Southampton on New Year's Day.
Reports have suggested that Manchester United have informally agreed to pay £40 million for the Chelsea ace although no concrete bid has been made - however Goal claim that the Blues are holding out for £45 million. Chelsea signed Mata from Valencia in 2011 for a fee of around £23.5 million.
For Manchester United a deal for Mata could spark a charge for Champions League qualification after a faltering campaign to date.
Under David Moyes, the Red Devils are six points adrift of Liverpool in 4th with their hopes of defending their league crown well gone. Should they seal a deal for the Chelsea star it would certainly give their campaign the shot in the arm it needs.
Chelsea have displayed a reluctance to sell to rivals in the past, but according to Goal would have to consider a bid in excess of £40 million.
According to reports there is a reluctance to hold Mata back on Chelsea's part - although they want the player to explicitly demand a move before sanctioning his sale.
Chelsea remain interested in Wayne Rooney, but it is thought that his future is not linked with Mata's, as was suggested in the summer.