Celtic were beaten 4-0 in the reverse fixture two months ago
Depending what happens in the other Group G match between Real Betis and Ferencvaros, the Hoops are likely to require a win in Germany to keep their hopes alive of qualifying for the knockout phase.
Postecoglou knows it is a formidable test against opponents who won 4-0 at Celtic Park almost two months ago, but – with his team unbeaten in nine matches since then – the bullish Australian has no intention of adopting a safety-first mindset.
Han sa: “We’ll only learn from the game if we play our football. You’ll learn absolutely nothing if you go out there and shy away from it. We can go and be conservative, try and hold on, try and survive, but if we do that we’ll learn nothing.
“All you learn is that you’re inferior and you don’t believe you can be any better. The only way we’ll learn anything in this game is if we go out there and take the game to them. That may be a painful lesson for us all but ultimately it’s a better way for us to find out.
“It may not be a painful lesson, it may show that we’re making a hell of a lot of progress. I’d rather go there and find out about ourselves than just go there and try to survive. That’s not the way you develop or grow as a team. If we want to get to these levels, we’ve got to use these games as a measuring stick and the only way to get measured is to go out and put yourself out there.”
Leverkusen have recently snapped out of a five-game run without a win and currently sit fourth in the Bundesliga. Postecoglou expects this to be his side’s toughest test since he became Celtic manager in the summer.
Han sa: “It will be a good challenge. They’re a good side. They’ve picked up their form in the league and are going well. We’ve played against them already so we know the quality of side we’re facing.
“To all intents and purposes, they are a Champions League team. It’s a super challenge for us away from home. There are also consequences on the result. When we played against them here, we did well in certain aspects but we also found that mistakes are going to get punished. That’s going to be the case in Germany.
“I’m sure we’ll have some bright spots, I still believe we can take the game to them, but there are going to be moments when our concentration is going to have to be at the highest possible level.”
Postecoglou has no fresh injuries to contend with, but Tom Rogic, Carl Starfelt and Giorgos Giakoumakis remain on the sidelines.