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.
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.
