The Spanish ace has impressed during his time Old Trafford after making the move from Chelsea, but the Reds have allowed him to enter the final year of his contract.
The 29-year-old has yet to be offered a new one and there is a chance he could leave on a free transfer at the end of the season unless a deal is hammered out.
Mata started the season well for United but has since struggled for action and reports have suggested he could be considering his future in the red half of the city.
City could also need a replacement for David Silva who, like Mata, is in the final year of his contract and has yet to pen a new one.
The Blues are confident that Silva, who has expressed a desire to stay, will eventually put pen to paper, but for now nothing is certain and Mata is being eyed as a replacement.