It's no surprise that ANY player struggles a bit more against better opponents, that is just common sense.
He certainly has room to grow and refine his game to be more mature in these big games. A little more patience is needed from him, which will come with experience. It felt yesterday like he thought every chance he had could be the only one, so he was trying the killer pass every time. With the way the game started, you can't really blame him for thinking this because possession was few and far between. But as I said, with more experience of these types of games he will realise that if he has patience, he will be able to spot when the opportunity is there for the killer pass rather than trying to force it at every opportunity.
I also think you need to factor in that it was his first United/Liverpool game so he may have been a bit overhyped for the game. Nobody will be more disappointed in his performance than himself, which you could see throughout the game.