I know Ndombele has had a frustrating season in terms of staying fit and constantly picking up little injuries, but the idea that he's not as good as like Gedson Fernandes is completely laughable. He was playing at a very high level in the Champions League for Lyon just last season - England and the PL can be a big adjustment for anyone. He's had a change of managers, philosophies and been reportedly feuding with a coach who isn't giving him a fair shake.
This is arguably a far worse situation than it was at Chelsea with De Bruyne and Salah. De Bruyne was a 21 year old from Genk, Salah was a 22 year old from Basel, both had performed well in Europe, but neither to the degree of Ndombele's performances last season for Lyon. They arguably were not good enough for the Chelsea first team while they were there, though I think Salah and De Bruyne certainly were by the time they were sold. There isn't a doubt that Ndombele is good enough for our first team now, he's clearly one of our most talented players. If Mourinho isn't getting the most out of Ndombele, that is a Mourinho problem, not an Ndombele one.