His technical ability is not very high. His passing is lacking and his on-the-ball ability is limited. I think he is a mid-table standard player, which is what the team is performing like at the moment.
I do not believe he will improve these. He is nowhere near good enough, and never will be, for a first-teamer in a title-challenging team. I feel mid-table is about his level. His ceiling is very low when considering past Manchester United midfielders since he will not improve much on his technical ability, which is what separates the best players from the average ones.
I can think of quite a few players that can do the same, or a better job, than McTominay, which really demonstrates that he is pretty average in Premier League terms. Neves, Longstaff, and Rice and others can all do that job.
People say he will be like Fletcher, but you have to remember he was always behind technically superior midfielders, such as Carrick, Scholes and Hargreaves. Fletcher only became first-team when Scholes retired and Hargreaves became a permanent crock.