I do think McTominay will get the nod over Amrabat as ten Hag likes to match aggression and energy with aggression and energy, which is what McTominay will be expected to deliver.
There can be none of this excessively low touches of the ball business. Actually, let me rephrase: if the touches are low then there has to be a large number of solid ball carries to compensate, so even if McTominay isn't doing well in passing chains or touch progression, he needs to be juggernauting up the pitch and stretching the opposition in that way whilst mentally conditioning them to the threat of him taking off, which would naturally drag bodies into his vicinity to snuff before he can pick up a head of steam, once he has them trailing, it's imperative he can release the ball - absolutely nothing fancy, just a short pass to another red shirt, please. On the other side of the ball, he needs to make a nuisance of himself. Positionally he is poor, but he should at least get himself in the thick of the action and be an obstacle rather than a traffic cone.
McTominay has a terrible penchant for ball-watching and letting players run off him. That can't be fixed, but instead of ball-watching, at least go toward it and be useful in some kind of press or pressuring sense.