Ndombele endured a disappointing first season at the London club following his big-money move from Lyon, scoring just two goals and providing four assists in 29 appearances.
The French midfielder has shown what he is capable of in the current campaign, finding the back of the net four times and laying on three goals across all competitions for a Tottenham side battling for honours on all four fronts.
"I have coached for so many years, had so many players, and I have enough experience to say and to feel that when a player is not playing very well, it's his responsibility, and when a player turns things around and brings his performance level to a very high level, it's also his responsibility," said Mourinho.
"It's a great example that with me the door is always open. The door of the team is always open, and when a player is not playing he has to try to understand why and he has to try to understand how can he walk through that door. He understood, he understood.