World Cup and European Championship winner Mata is no longer a first choice.
But after victory at Hull City, Mourinho said: "If the game goes on being 0-0, then Mata is coming into the game as he is a fantastic player.
"But the game went in another direction, we scored, and then after that my brain worked in another way.
"However, we need them. We need Mata, we need Schurrle. We need these players as we cannot do it with just three.
"It is just a moment. Maybe in February some are not in the best form and Juan is coming at the top of his game.
"Will Juan be here after the transfer window? Of course."