Huddersfield produced a marvellous performance at the John Smith's Stadium on Saturday, scoring twice in quick succession around the half-hour mark to give United a mountain to climb.
Goals from Aaron Mooy and Laurent Depoitre ultimately proved unassailable in spite of Marcus Rashford's late header, and Wagner was elated after ending a six-game winless run with a famous triumph.
"I said to the players that I wouldn't expect a result, but that I can believe in a result," he told reporters.
"That is why it so huge for us. It's a very proud moment, that we were able to beat Manchester United at home.
"In one game you can have a chance. If there is a chance, try to catch it. Today was our moment because we were at our best and maybe they weren't.
"Small Huddersfield have beaten Man United and it is one of the proudest moments in my managerial career. It makes me very happy.
"It's another chapter in the fairytale we started nearly two years ago. It has shown everything is possible in football.
"I have to make totally clear what are extraordinary and ordinary results for us. This was extraordinary, but we must stay humble."