He was very good against Leeds, but I will be a bit more reserved in rating his potential. You have to take into account that Leeds left an absurd amount of space in midfield; I don't think I have ever seen so much space before. McTominay used it very well, but this question remains: is he going to get that type of space in most Premier League games? The answer is no.
The question still stands as to whether he can have such a creative/goal scoring impact against teams that will not provide such space. I feel this performance is more of an outlier from the norm of what he has shown to date, rather than a promise of what is to come. He scored 2 and assisted 1, but this is not the norm as these are the only direct goal contributions he has this season. He also did not score since March in the Premier League, and before that October of last year. It has not been a consistent aspect of his.
I'm not saying he could not improve his goal output, but I do think that people are perhaps expecting too much based on yesterday's game. I don't think he has once come close to replicating yesterday's performance in a match beforehand, so I think we need to take it with a pinch of salt in terms of his potential.
I mean, I see some people saying he could end up better than Carrick, which is a bit over the top based on this game. Especially, since this game was really based on his attacking output rather than any significant defensive contribution. The defensive side was mostly left to Fred, with McTominay making no tackles etc. because he was pushing further forward, so I don't really understand the comparison to Carrick if he is to play that role.
I still think he can be a good player, but I will wait to see whether he can ever get close to this type of performance again when we play more compact teams.