Mustafi may have been a bit slow to react & cover there, but in general he was good again.
And with Kos playing at this level every week, we can be quite happy with our D apart from the left side. And the understanding between the CBs has been growing steadily (the second goal was 99% Monreal being unreal; just as Cech could have done better with the first maybe, but it was all on Xhaka), so there's more to expect. At least to me this is the best we've had in many, many years.
There's one extra which Mustafi gives us as that he can sometimes be the extra man from behind. Yes, Kos sometimes is in the mood to go there, and even Per in the past did a bit of that. But Mustafi is a guy who really feels comfortable going forward, or going anywhere, for that matter.
As for the ref - yes, he could, but he's a real prick for thinking this was a straight red. It's like you don't know what the context really is - this is just not given in the PL (and I think it's good it is like that). Was a stupid foul to commit, but shouldn't have been a red. But ref was a complete ass for the entire afternoon.