And although the transfer fee is a huge increase on the £23.5m ($NZD 38 million) Chelsea paid Valencia in 2011, Moyes is keen to land a big signing in the transfer window.
Mata is determined to push through the move and Mourinho is understood to have accepted that he wants to leave in search of first-team football.
The midfielder has concerns about his inclusion in Spain's World Cup squad this summer if he is not playing regularly for his club side and Moyes will give him that opportunity as he starts building a team for the future.
United are treading carefully only because Moyes and his chief executive Ed Woodward failed to sign the likes of Thiago Alcantara and Cesc Fabregas during the summer.
But this time they have been given all the encouragement they need that Mata wants to leave Stamford Bridge.
Chelsea's Player of the Year for the past two seasons has not featured in the club's last three games. After he stormed off at Southampton on New Year's Day, Mourinho admitted he would listen to offers, prompting United to make their move.