The former Barcelona shot-stopper has only made two first team appearances for United since joining the club last summer.
The 33-year-old has found himself frozen out of the United dressing room, a situation which Mata describes as disappointing.
"It is a shame what has happened with him as he is a great goalkeeper," Mata told Onda Cero.
"He wants to compete and has proved himself as a goalkeeper. I don't know what happened between him and Van Gaal, that is something only they can say."