The Belgian side trail the Red Devils 3-1 ahead of the second leg of the play-off and Preud'homme admits a 'miracle' and a 'perfect performance' are the only things that will see his team through to the group stage proper of Europe's premier club competition.
"Miracles happen sometimes, but they are two goals in front and they are a very professional team," he said.
"To get a miracle, you need to be perfect and you need the other to not play at his best potential. We have a lot of players out, too.
"If we want to have a chance, we have to be perfect. It will be very difficult, but let's pray."