The Red Devils take on the Belgian side in the second leg of their Champions League play-off on Wednesday night having got the better of them 3-1 at Old Trafford last week.
But van Gaal is aware that Michel Preud'homme's men have the ability to turn over such a result in front of their expectant supporters at the Jan Breydel Stadium.
"We are still not there yet, it's very important," said the Dutchman.
"We have to play the game, at the end you have to have scored more than one goal than the opponent. They have beaten Greek opponents 3-0, they are capable of doing that with the support of their fans."
Van Gaal added: "I don't think my players will take it easy.
"We have had the same preparation and I don't think they shall start differently as last week."