Van Gaal said that in football there are no guarantees, but with the size of the club and the expectation at United he has no choice but to put everything into the job.
"I will do my utmost best - that's what I can give," he said in his first press conference as Red Devils manager. "I cannot give any predictions because you never know.
"It's the biggest club in the world - in two days I already know how big United is and how big the supporters are. I have to prepare a team and I have to adapt to this big club, [but] it will not be easy. If you see my career, you can see what I have won.
"Every season you have to prove [yourself]. Last season you are seventh, so then you are not the biggest club. When I was in China and Brazil, people were talking about United when I was coach of the Dutch team. There is a lot of expectation."
Van Gaal has also managed Barcelona and Bayern Munich.