Mata has made just three appearances this season and was omitted from the Blues squad to take on Fulham on Saturday, with Mourinho warning that the Spaniard must adapt his game.
Grant is sure the Special One has his reasons for dropping Chelsea's Player of the Year in each of the last two seasons, but admits he thinks it is a strange move.
"I personally don’t understand [why he left Mata out]," Grant told reporters. "But in this position Chelsea have so many players, so if Oscar, Eden Hazard, or Willian are out, people will ask why they are out of the squad or team.
"A manager needs to take a decision and in this case it’s a tough decision. Jose will know how to manage them well."
When asked if Mata still has a future at Stamford Bridge, Grant replied: "I think yes. He’s a great player, he’s won games, gave assists and scored goals."
Blues owner Roman Abramovich has been accused in the past of interfering with the club's transfer policy and team selection, but Grant insists this is not the case.
"He didn’t do it [with me] and I don’t think he did it with any manager," he added. "This is a story that isn’t true. At the end of the day the manager made the decisions and he never said do this or that.
"I’ve known him a long time and he never took a decision against me. If a manager said 'I don’t want this player', he will not buy him."