"We found that when we won the trophy two years ago. Supposedly smaller clubs like Rapid Bucharest and Slovan Liberec really gave us a hard game. "It's like a trap for the big teams, we have more to lose than the smaller ones. We want to go through and qualify.
"Look at Chelsea, they know what can happen and have lost to smaller clubs in Europe. You have to have the right attitude, make sure the focus is top class, and you must give more application in what you do because these are not easy games to play.
"The way you approach the game, how you prepare physically and mentally are critical, that's what is important at this stage of competitions. If you have all that, it takes away a lot of the problems that can occur."
It is unlikely, of course, that the Reds will be short of motivation when they step on to the pitch in front of a capacity crowd tonight after bristling criticism from Olimpija manager Saud Besirovic.
Houllier added: "They gave us a good game in the first leg, their keeper made a lot of good saves. Whatever their manager is supposed to have said about our players, he obviously does not know them. Because if there's one thing about our boys, it's that they have heart and guts. The best way to answer such things is out on the pitch.
"The opposition has nothing to lose, they will raise their level like they did in Slovenia. That is probably why their manager says he would like to play us 20 times in a season.
"We need to get through two rounds to take ourselves into 2004 still involved, that is what we want because our reputation is built on Europe and what has been achieved in the past. We must maintain that."