Predicting which midfield players will score goals at PL level is notoriously difficult. Nicky Butt got plenty of goals for the reserves. Tom Cleverley had no trouble scoring goals when on loan in the Championship yet barely made a clean contact when presented with a chance to score for United. Schneiderlin got 5, 2 then 4 goals for Southampton in the PL yet just one for United with 0 assists.
I have absolutely no idea if McTominay will turn into a United standard midfield player by the time he is say 24 or 25, when a decision has to be made to keep him or let him go. Will he be a player who makes costly errors in our box and fails to finish rare chances in the opposing area ? Or will he be the opposite, few defensive errors and capable of getting the occasional goal and some assists ?
As things stand, I'd be perfectly happy for him to replace Carrick as our 5th choice central midfielder next season, but would need to see a larger sample of games before I was comfortable with him replacing Fellaini as 4th choice central midfielder. Of course, Mourinho sees him in training so is in a far better position than us to make these judgement calls.