Mata, 25, has played just 17 times under José Mourinho this season and started 11 of those games. In his last appearance at Southampton on New Year's Day, he reacted unhappily to being withdrawn in the 53rd minute with the scoreline at 0-0, yet Chelsea went on to win 3-0.
Mata was named player of the season in his first two campaigns at Stamford Bridge, as Chelsea won the FA Cup and Champions League under Roberto di Matteo, as well as the Europa League under interim coach Rafael Benítez. However, Mourinho's arrival has seen Mata demoted in favour Willian, who joined in the summer for £32m from Anzhi Makhachkala. Mourinho stressed earlier in the season Brazilian forward Oscar was now his preferred No.10 and has lavished praise on the more disciplined Eden Hazard in recent weeks.
Spaniard Mata is also believed to be concerned at a lack of game time ahead of this summer's World Cup. A member of Spain's triumphant squad four years ago, as well as their 2012 European Championship success, the former Valencia forward faces stiff competition from La Roja's array of midfielders, and a reunion with Benítez at Napoli was touted earlier this month. Atlético Madrid have also been linked with him.
United are loath to panic-buying in the winter window, but their seventh Premier League defeat against Chelsea re-emphasised the numerous deficiencies in their squad, and Moyes has identified Mata as an ideal acquisition who could have a similar effect to Mesut Özil at Arsenal.
Although United are arguably in greater need of central midfielders, wingers, a left-back and a centre-back, Mata would add some extra stardust to an attack that will have Robin van Persie and Wayne Rooney back from injury soon. Moyes' reliance on the prodigious 18-year-old Adnan Januzaj was again apparent at Chelsea, and Mata could ease the burden on the Belgian in the absence of Van Persie and Rooney. It would effectively kill Shinji Kagawa's United career, though. A similar player to Mata, the Japanese was conspicuous by his absence from the starting XI at Stamford Bridge on Sunday and has hardly impressed under Moyes.
Chelsea should make a hefty profit on Mata, who was signed for £23.5m in 2011. United's record transfer buy remains the £30.75m they paid Tottenham for Dimitar Berbatov in 2008 and it is likely they will have to offer close to £40m for Mata. United are under pressure to spend after they signed Marouane Fellaini in the summer for £27m.
Last month, Chelsea announced a loss of nearly £50m in the year to June 2013 and have already sold Kevin de Bruyne to Wolfsburg for £18m this month. Nemanja Matić has returned to the club for £21m, but the sale of Mata, who is halfway through a five-year contract, would ease any anxiety about financial fair play.