His side ensured their name is in the hat for tomorrow’s quarter-final draw thanks to a relatively straightforward 4-0 win over Championship opposition.
The Australian boss, however, was far from happy with the first-half performance and was visibly frustrated midway through the first 45 minutes.
“I was really disappointed with the way we played,” he said. “We didn’t work hard enough, didn’t have enough urgency in our game, too many touches.
“We were comfortable enough but that’s not what I’m after.
“The second half was better, we got the tempo up and the substitutes made a difference and we’re through to the next round.
“I got frustrated because I shouldn’t have to remind anybody about these things. We’ve been building the foundation for so long and you’ve got to work hard and buy into the kind of football we want to play.
“Without making allowances, there are mitigating circumstances. I made seven changes so I knew, particularly at the start, we would lack the fluency we’d been showing recently but it was important we got some minutes into some of the guys because we’ll need them going forward.
“The way we set up and play our football, I forgive everything from mistakes, people making bad decisions. But I just won’t cop people just putting in a 45 minute shift. That’s not what it’s about.
“That’s when my frustration comes out. I just feel every time you put the shirt on there’s a price to pay for it. That price is you give everything you have for the time you are out there, within the context of how we play."
When asked if his team would have plenty left for the Conference League clash with Bodo/Glimt on Thursday, Postecoglou added: “They should be able to go again because they only played for 45 minutes as far as I’m concerned.”
Meanwhile, Christopher Jullien made his first Celtic appearance in 410 days as a second-half substitute.
Postecoglou said: “He’s worked hard in training and, like every footballer, he wants to play so we gave him some game time and I’m sure that will encourage him to keep working hard and take his opportunity when it comes.”