The Red Devils saw off the Spaniards 2-1 at Old Trafford last night thanks to Marouane Fellaini's late winner.
Despite ending the summer with six victories from six matches, Van Gaal believes that his side looked sloppy against their La Liga opponents.
"The main thing is that we have won in spite of playing our worst game. We made a lot of wrong choices, wrong passes and we didn't press too much," he is quoted as saying by Goal.com.
"I think Valencia is a very good team, very comfortable on the ball so it is not easy to press but, in spite of that, we dominated the game and at the end we are the winner because we are fighting until the last minute and that is also a good signal."
Van Gaal takes charge of his first competitive game as United boss on Saturday when Swansea City travel to Old Trafford in the Premier League.