The Spaniard can leave Old Trafford as a free agent at the end of the season and begin negotiating with foreign clubs in January.
But there a still a number of issues that must be ironed out with Mata's agent, his father Juan Snr, according to a source quoted by The Sun.
The source said: "Mata's dad is a tough negotiator and it has not gone down well how this situation has been allowed to drag on.
"United will try their best to convince them to stay, but it is by no means certain that even extra cash will do the trick.
"It's purely about playing for Juan. The club know they have left it late and are desperately running out of time.
"They are playing catch-up because of various contract issues and the problems on the pitch being a priority to sort."
Mata has been linked with a return to former club Valencia.