#2 He isn't as adored as he once was
He was certainly Arsenal's biggest star, which is why there was such an onus on him to come up with the goods. It would appear that this onus helped him because he was the one to supply what his fans wanted more often than not.
At Manchester United, there are the clear fan favourites. The main one of those being David de Gea, at the other end of the pitch to Alexis Sanchez.
While I am by no means saying that the United faithful need to chant his name every week, showing him some more love is definitely a technique to consider. The power of the twelfth man could really play its part here.
He's a man who likes to make up for the love he's given. There's a reason why the Arsenal supporters made a banner of his dogs...