I obviously agree with you regarding the poor refereeing.
I do still think he was bullied out of the game to the extent that he wasn't doing the things he has been doing. He wasn't really taking players on, or getting into dangerous areas, and he didn't present that much of a goal threat. Yes, he had several shots, but most were speculative efforts from distance that rarely looked like troubling the keeper. The rushed finishes that you mention indicate that he felt under pressure, perhaps as you suggest because of the weight of expectations, but also I think because he felt that a heavy challenge would come in if he took any time on the ball.
If the opposition think that a player can be marginalised by a bit of rough treatment, then you can be fairly sure that it will be meted out. Mason Greenwood still has some work to do in developing strategies for dealing with this kind of pressure. As an aside, his team mates also need to step up: if a player in the first team had suffered the same foul early on that Greenwood did, the whole team would have reacted to it and made it very difficult for the referee not to issue a card. It may be described as gamesmanship, but players do need to look after themselves and each other on the pitch, and making sure referees take action against bad fouls is all a part of that.