Mourinho spoke about Ndombele after the midfielder had to be withdrawn 25 minutes into the match:
"He's always injured. He's injured, he's not injured, he plays one match, next week he's injured, he plays another match. We are full of hopes.
"This is it since the beginning of the season. Of course, it is a concern.
"You think you have a good player, you think he is in an evolution process, he plays very well against Norwich, you are full of hopes that today he is ready for it.
"And he is not ready for it. I cannot say much more than that. It's a situation from the beginning of the season."
Spurs signed the 23-year-old from Lyon in the summer in a club-record £53.8 million deal.
He has made just 14 starts in all competitions for them so far, though. Ndombele's outing against Norwich City on Saturday was only the third time he's completed 90 minutes in the Premier League, and the first time since August.
Tottenham have played 11 domestic and European matches since Jose Mourinho took charge on November 20, and the Frenchman has not even made the squad for five of those.
Football commentator Rob Daly observed the frustration at his substitution on Wednesday, given how impressive he has been when he's been on the pitch:
The Athletic's Alex Kay-Jelski also singled him out as an important player for the Lilywhites:
Alex Kay-Jelski @AlexKayJelski
4) I have mixed feelings re Toby. Still don’t think he’s anywhere near as good as he was and would have liked to have seen a Foyth/Sanchez pairing given proper time 5) Ndombele looks class. Once he gets used to the pace of the league, he will be someone to build a team round
Given the level of his performance and the way he has been in and out of the side, it's understandable Mourinho would be frustrated at Ndombele's inability to play more regularly.
However, as football writer Grace Robertson noted, the player is evidently dealing with an injury and could exacerbate it if he meets the manager's playing demands:
Injuries aren't something that have particularly troubled Ndombele prior to joining Spurs. He made 96 appearances for Lyon in two seasons, most of which were starts.
Although it may mean doing without him for some time, Tottenham might be better off letting him take the time he needs to fully recover from his fitness issues, so that they might be able to deploy him more consistently once he's ready.