He says: "I think the frustration of us not winning for five games made us think 'where are we going to secure a clean sheet?' for one, and secondly, 'when will we score goals?'
"The answer was the least likely of places, but it was a fantastic performance.
"To be fair, as a team we have been very organised in the last three games but within that, if it's not a contradiction, when we haven't been we have conceded goals.
"Possession wise I thought we were fantastic but we didn't create clear cut chances, and that is the strength of Manchester United. They played well and that is what makes our victory all the nicer."
Despite the win, it was probably David's busiest performance in a West Ham shirt, and he says: "There were a couple of times at Old Trafford when we lost our shape but I was able to make saves.
"Manchester United are a good side and they are going to create chances so it was just about being able to match what they had.
"There was no best save, that is what I am there for, and had they scored then arguably the saves made are not worth so much, so I was just happy with the performance."
He says people rule Manchester United out of the title race at their peril, and adds: "They are the champions of England and regardless of what other people say they are still a great side.
"There are too many games to say they are not going to be in the shake up - I wouldn't write them off at all."
David knows it doesn't get any easier with the visit of Arsenal at the weekend, and he says: "It still is a tough month and the win doesn't change the fact that the rest of the games are going to be as hard as that.
"What we can do is take a bit of confidence from it, but at the same time keep things level and not think because we have beaten Manchester United we are any better than we are."