I don't think it's people saying he can do all this now though. People are looking at a kid coming through that they think, is playing within himself at his managers instructions. That's a massive mental skill to have as you need to constantly remind yourself to keep it simple and stick to the task given to you.
The hope is that he pushes on from here once Mourinhos trust is gained. It's not like he's been full of sloppy touches, constantly giving the ball away and getting bullied in the middle of the park. He has held his own and not looked out of place. The test now is whether he can move past this to cement his place in the coming years, or whether he will stay as the steady midfielder.
Also, at the moment, there is a case to be made that a steady midfielder is just what we need. Matic is our ball winner and controller, whilst Pogba performs the more creative task. McTominay is being asked to provide support for both and so far is doing fine in allowing them to perform better. Time will tell if he can make the step up from supporting others to having his own impact on the game.