The Spanish playmaker has only been used sparsely in the Blues' three opening Premier League fixtures, and Willian's imminent arrival from Anzhi Makhachkala will result in even greater competition for attacking-midfield places in Mourinho's lineup.
However, the Portuguese tactician has insisted that Mata will not be leaving for reported admirers Tottenham Hotspur or Atletico Madrid, and revealed that the 25-year-old's lack of game time is down to a fitness issue.
Speaking after Chelsea's 0-0 draw with Manchester United, during which Mata was an unused substitute, Mourinho told a press conference: "These are nonsense stories. He is going nowhere."
Mata has apparently been the subject of enquiries from Spurs and Atletico, who were hoping to persuade Chelsea to allow the former Valencia star to return to La Liga on loan.