It's very clear that Arteta has coached (to some degree) the Man City/Pep blueprint into the Arsenal players. All the player movements, the channel and hole drop-backs, wing-rotation, wide players holding very wide. Someone here commented that Arsenal will eventually run out of steam, but I think it's more important to realise that Arsenal are only so effective in the press, and relentless, because they're keeping the ball much better and pressing from a much higher position on the field. It's infinitely less tiring to press in the final third and keep the team compact high up the field, than it is to press from the halfway line and find your forward line stretching the gap between itself and the midfield.
If anything, it's more likely that United will tire before Arsenal. It's harder to chase possession teams than it is to be a possession rushing to recover possession.