we both know those players have the 'basic skills' to do much better than they did last night.
so it's other factors.
Tierney's was probably just a one-off human error, maybe compounded by his only just returning to match fitness.
as for Auba, I personally would have started Laca yesterday and kept Auba for Citeh.
Arteta's far too prone to just keeping players on the pitch when they're doing well and running them into the ground.
hat-trick or not, he should have rested Auba last night for Laca.