Mata helped Galatasaray win the Super Lig title last season, one year after he left United.
Juan, 35, was a firm fans' favourite during his time at Old Trafford and returned to Manchester for the Manchester International Festival last week.
On Saturday afternoon, the Spanish maestro posted a heartfelt goodbye to Galatasaray on Twitter.
I am not sad because it ended, I am happy because it happened. ud83dudc9bu2764ufe0fud83eudd81ud83cudfc6ud83eudd47
To you, Cim Bom fans:
I am so proud to be part of this family forever. The atmosphere that you create in our stadium and our away games is second to none, I can tell you that.