The Spain international, Chelsea's player of the year for the past two seasons, has been strongly linked with a move to Old Trafford after falling out of favour following the return of Jose Mourinho as manager.
Media reports on Wednesday suggested a fee in the region of £35 million had been agreed between the clubs, with the Spaniard's move to be finalised swiftly.
However, Moyes was in no mood to shed any light on the situation in the aftermath of United's dramatic League Cup semi-final defeat to Sunderland - sealed in a penalty shootout.
Mata has made just 11 Premier League starts for Chelsea this term, but now has the chance to relaunch his career with the reigning champions, who are desperate to improve on their current position of seventh.
United were widely criticised for a lack of activity in the previous transfer window, when Guillermo Varela and Marouane Fellaini represented the only new arrivals at the club.
Moyes, who succeeded Alex Ferguson at the end of the 2012-13 campaign, has also come under pressure due to his side's poor league form and will hope Mata proves the catalyst for a revival in the season's remaining months.
Mata joined Chelsea from Valencia in June 2011 and has scored 33 goals in 135 appearances for the London club.
‘I don’t think Liverpool would look at Ollie Watkins, a striker isn’t a pressing issue for them – it’s Arsenal who need one’ Former Reds star explains why his old club don’t need an out-and-out forward this summer
‘He’ll still be playing at 40 at a good level because he’s in such good shape and looks after himself so well. He does everything to be a top professional’: Ex-Liverpool coach insists Mohamed Salah has plenty more miles in the tank
