Having lost 2-1 to Bournemouth last week, United were condemned to defeat by the same scoreline against another newly promoted side, Norwich City, on Saturday.
The result means United have now slipped outside the top four and the former Holland manager admitted the lack of belief within the ranks was alarming.
“I can only say what you have seen today is what a lack of confidence can do to your team,” said Van Gaal.
“It was not good enough and although it is our first league defeat at home it is also confirmation that we are in a very bad period and we have to come out of that.
“That’s difficult otherwise we would have done it today. You have to win against Norwich and win every home match and we have done that until today.
“The problem is how we manage to come out of this bad period. It can be done when you are sticking together not only players, manager and staff but also fans because that is very important.”