You have to wonder how much Arsenal is playing on Pep's choice here.
Our defense is decidedly slower, more physical. You could argue Stones needed a rest, he makes at least one error leading to either a goal or goal scoring opportunity each game, however it's possible he's being saved for a more technical, faster Arsenal.
In the end, our defense was always going to be shakey tonight regardless. It's up to the forwards to actually start turning our chances into goals to make the difference. I agree with Nolito starting, he's earned it, but I'm not convinced he and Yaya will be enough.