Valencia and Shaw have been very poor, which is unlike them defensively.
I'm not even surprised that Valencia has been poor. He's finally being exposed for the lack of defensive qualities that he has. This is what we badly miss when Rafael (or at least a proper right back) isn't playing for us.
I wouldn't mind seeing Valencia come off for Rafael at the moment. Valencia has been a huge weak point in our defence. He's been a big contributor to Monreal's goal, and every time Arsenal attack, Valencia makes it easier for them to do so.
Shaw settled in well after a shaky first period. He's starting to get into grips with Ox, and we just need Rafael to come in for Valencia to strengthen our right side.
di Maria has been very lively and dangerous so far as well as Rooney, even if Rooney's not been involved. Fellaini's having a below average match at the moment, conceding too many fouls and being very poor at winning the ball. He's been quite meh going forward as well, which I'm not surprised with, and he's been at his best when he's been on the receiving end of the far-post crosses from di Maria.
Herrera and Blind need to show more discipline in their game. They need to stop pressing Arsenal, else it will be easier for them to open us up. Young has been quiet in the match so far, barely getting involved going forward whilst doing much of his work off the ball.
We need to press less and keep ourselves organised if we are to stop Arsenal. Welbeck's been quite sloppy and ineffective so far, but he'll start to become more dangerous if we keep dragging ourselves around. Sanchez is giving Valencia a horrible time at the moment, and we need our midfielders to stop getting dragged out wide too easily. We can win this match if we play with more control and less chaos.