goon
I think the reason he's reluctant to bring ESR/Vieira on when we're winning is because it only really changes the team dynamics for us if it's for Xhaka.
And once we go ahead, it's very difficult to take him off because he's so much better defensively than those two. We weaken ourselves in every defensive phase, and also hugely at set-pieces - which can come thick and fast at that stage of the game.
At the same time, Xhaka is one of the guys who struggles most when the game becomes chaotic, when he's pushed deeper with less time on the ball because the opposition go for broke in their pressing.
Ultimately, I think it's an indication that Arteta doesn't yet trust this team to pass with composure and authority to see the game out. Which he's not wrong about, and I could very easily imagine both ESR and Vieira wilting badly, just like a lot of the others, in the face of the sort of onslaught that is common place from a home team trailing in the Premier League. So, the point being, we're likely going to be almost exclusively defending anyway.
This is the Xhaka issue viewed in micro, the macro view is his place in the team to begin with. I don't think he'll be in there when Arteta has built the team he wants, but for now, he's critical to making it work.
