The youngster has been called back from Vitesse Arnhem by Mourinho to provide cover across the back-four.
Kalas said: "It was the first day of training when Mr Mourinho came to me and asked me if I would accept the challenge to be a first team player and give it everything and, of course, I accepted. I had no doubts. Straight away I said yes. I played for two years in Holland, I played 63 games and another maybe would be worse than better."