Old Trafford has sounded like this before, but not often enough in recent seasons. The second half was a reminder of what the ground can do when the team gives it something to hold onto.
United were second-best for half an hour, then scored twice in six minutes and never looked back. The press jumped a yard higher. The crowd went with it. Arsenal, suddenly, were the side chasing shadows.
It will take more than one night to rewrite a season. It is enough, for now, to remember what a statement win feels like.