Mata has been linked with a move away from Stamford Bridge ever since Mourinho completed his return to the Blues - with Arsenal and Manchester United reportedly showing an interest in the 25-year-old.
Mourinho has since refuted speculation that the Spaniard, who was Chelsea's Player of the Year last season, was surplus to requirements at the west London side despite bringing in attacking midfielder Willian for £30million.
With news of the player's apparent unrest, reports suggest Barcelona are set to make a move for the former Valencia star.
Mata has so far been limited to a 65-minute appearance against Aston Villa as Mourinho has opted to start Oscar, Eden Hazard and Kevin de Bruyne behind the main striker against Hull and Manchester United.
The Spaniard will be hoping he is able to prove his worth to Mourinho on Saturday when Chelsea face Everton at Stamford Bridge.