United were second best for much of the game in atrocious weather at Old Trafford on Wednesday, but still secured a fourth successive victory thanks to Juan Mata’s 83rd-minute free-kick.
It was tough on Watford, and on their head coach Quique Flores, who coached Mata in Real Madrid’s youth set-up and at Valencia.
United boss Van Gaal, though, felt it was just reward for the way his players battled against physical opponents.
“I don’t think it was an ugly win because the free-kick was a fantastic shot from Mata,” van Gaal said.
“The performance was not so good. The weather meant it was difficult to play good football. It was a physical game.
“If you see the size of the Watford players compared to ours, it was not beneficial to us.
“I think that the youngsters give us a lot of spirit and now when the dressing room is full of joy and all the players are laughing, that is beautiful to see.”
United’s teenage striker Marcus Rashford was unable to repeat the scoring form that brought him two goals in each of his first two senior games for the club.
He laboured in a central striking role in the first half, but was much more effective when moved to the right in the second half, with Anthony Martial switching inside to take the central role.
“In the first half, Martial was not in the game as a right winger and Rashford was not in the game as a striker,” van Gaal said.
“That’s why we changed the shape. From that moment, we were the better team.”