Mata has been offered a new contract at United, but has decided that it is time to leave the club.
The 30-year-old is under contract at Old Trafford until the end of the season.
Mata featured in United’s first two Premier League fixtures this season, but was not even part of the squad for the game against Tottenham.
Mata has been a key player at United since joining the club in a £47million deal from Chelsea in 2014.