The Spanish playmaker has only been a Red Devil for just over 12 months after signing from bitter rivals Chelsea in a then club record £37.1m deal.
However, his future at Old Trafford has been far from clear since Louis van Gaal’s arrival, with the Dutchman having splashed out huge fees to land the likes of Angel di Maria and Radamel Falcao while shifting to a 3-5-2 formation, limiting Mata’s chances and effectiveness.
This is believed to have alerted La Liga champions Atletico, who are keen to lure the pass-master back to his homeland.
United originally wanted star 22-year-old Koke to be thrown into a potential agreement, but the Rojiblancos have no desire to lose their youngster.
Reports state that £22m plus the Brazilian defender has been put forward for Mata, who has not started a game since January 17.