Is it hard to understand that McTominay came into the first team under a special circumstance, and was instructed by his manager to play in a certain kind of way? Nothing more, nothing less. A greenhorn, playing before large crowds and donning the jersey of a big, big club, was not daunted by the occasion and carried his manager's instructions to a T. We don't know, nor do we have any way of knowing, if he is capable of more. That test will come. If he is found wanting then, he will fade away, like scores of others before him.
Until then, give him some breathing space. Laud him for what he has done, and wait for better things to happen. Jump on him when he becomes a regular trooper, benching others more deserving of the spot, and plays badly.
Those zombie critics above, are they United supporters? I sometimes wonder.