The former Australia boss made an instant impact at Parkhead after arriving last summer by winning the Premier Sports Cup and wrenching the cinch Premiership title back from Old Firm rivals Rangers.
The Hoops go straight into the September group stages of the Champions Legaue, with the draw taking place in August and Postecoglou is certain about how he will feel on match-day one.
He told Sky Sports: "Again, it's brilliant. I remember when I was at the World Cup in Brazil.
"I'm sitting on the touchline and we played against Chile in the first game and I remember thinking, 'what the hell am I doing here? I should be sitting in the stand watching.'
"I felt like someone was going to tap me on the shoulder and say the game is starting, get off the pitch.
"It's going to be the same with the Champions League.
"I know there will be a moment when the whistle goes and I think 'what am I doing here?'
"It doesn't feel like this should be happening but again that's the beauty of it.
"I've never doubted my ability or my belief that I could perform at that level, but how can you chart that course?
"You can have that dream of winning Wimbledon or the Open and you can do that with performances and working hard - it is not easy mind you - but as a manager you need other people to take a chance on you to make those steps.
"Obviously you've got to take every opportunity that's given to you, but there isn't a manager who can say 'in five years' time I'm going to be coaching in the Champions League.'
"That doesn't happen, certain things have got to happen.
"The fact that I'll get that opportunity is a great weight of responsibility because that's where this football club wants to be and needs to be. It's also just the opportunity I've always craved for myself."
Postecoglou believes his instant success at Celtic will make it easier for the club to appoint coaches in the future who may not be household names in Scotland.
He said: "I remember when I was national team coach of Australia, the first Australian who had been put in charge to get us to a World Cup.
"I knew that after five World Cup campaigns with foreign coaches, I knew that if I didn't make it to the World Cup or didn't win the Asian Cup another Australian coach wouldn't get an opportunity for another four or five cycles.
"That manifests itself to over here where you know that absolutely, if I don't have some sort of impact it's very unlikely someone will get a door opened from that side of the world.
"It's not even from that side of the world, even say Europe maybe if someone is a little bit unknown or has a body of work that shows they have ability but doesn't have the profile, again then people aren't going to take than gamble."