I don't think Jonny Evans would have solved any of our problems. I had always had the impression that he's pretty slow, weak and vulnerable one-v-one and having watched him a couple of times recently, I still think that.
He's obviously a very, very good reader of the game and unusually confident and competent on the ball, but a lack of power means he can't be dominant in the way that most good centrebacks can. I also think he benefits massively at West Brom from having a workhorse midfield in front of him and two physically imposing players in Dawson and Hegazi either side of him. The former is basically a third centreback next to him. It would be a very different experience at Arsenal with Xhaka in front and Hector and Mustafi ether side!