I know I'm in the minority here and it might not 'go down' well, but personally I think Son was genuinely injured and had been caught quite nastily by McTominay.
I remember Jermain Defoe getting a booking against Ukraine in a World Cup/Euro qualifier for 'elbowing' a player behind him while shooting for goal, the difference being it was natural movement in order to shoot. That guy went down as well.
I've looked at the replay several times and to me McTominay deliberately slaps him at close range and the movement of his arm/hand didn't look consistent with him running with the ball. If he'd just used his arm statically to hold off Son it would be different but that's not the case.
It's difficult to tell for sure whether he poked his eye but it didn't look good to me. In real time it looks a bit soft but you can't really tell from that angle; however, if you look at the second replay, he is definitely caught.
One significant thing is that Son went straight down; it's not like he belatedly did that when Man Utd scored, and that itself was several seconds and three passes later.
Some of the criticism/allegations he received afterwards, especially from OGS, were pretty disgusting to be honest.