Van Gaal opted to put the teenage winger in an unfamiliar role despite also having the option of playing Shinji Kagawa, who remained on the bench.
Kagawa was tested in the role during the pre-season tip to the United States, and the United boss reckons Januzaj offers more inventiveness in the position.
United travelled to the Stadium of Light without injured midfielders Michael Carrick, Marouane Fellaini, Ander Herrera and Anderson.
Van Gaal said: "It is not usual that we have four injuries in midfield. Every club that has that has a problem.
"That is why I played Januzaj in midfield, because as a coach I want to win.
"I have said that we need creative passing and I thought Januzaj could provide that. That is the reason.
"Kagawa, I have tried to play him in that position in the United States and he could not fulfil my wishes and my philosophy.
"We have spoken about that and he is more of a 10.
"Juan Mata was playing at 10 and I thought I had to change the other players, which is why I chose to bring on Januzaj.
"It is because of creation. Kagawa can also create, but I asked Adnan to play there because of the lack of midfielders.
"When he played in Belgium, he played in midfield so I asked if he could do it for us. I said to him 'you are on the bench as a midfielder' because I want him to focus."He played there and you could see it's not so many times that he's played there. But he did his utmost and I cannot demand more.”