Former Spurs striker Troy Parrott was the difference as he forced Lucas Bergvall into an own goal as AZ won the first-leg 1-0 of their round of 16 tie.
Postecoglou said afterwards: "Yeah, it wasn’t great from us tonight. Pretty much all facets of the game. We didn’t really get to grips with any part of the game. We struggled to gain any momentum with the ball, we weren’t as aggressive as we needed to be without the ball. It was a tricky surface but having said that we kind of knew that from yesterday and yeah we just didn’t tackle the challenges we head out there in a real positive way. That allowed AZ to get into a rhythm, which when you’re playing away from home, it is difficult to stem the tide.
"Look, it’s a combination of things, but we’ll analyse it and have a look at the areas where we struggled with, but it’s fair to say yeah it was nowhere near the level it needed to be."
Pushed about the attitude of his players on the night, Postecoglou dismissed any criticism, despite admitting the performance had fallen short of standards.
"I don’t think it’s a matter of effort or attitude. I don’t think it is going out there and not trying, but like I said we didn’t really come to grips and have the right mindset to tackle an away fixture in Europe. It is always tough and we obviously conceded the goal, which was a disappointing set of events but even after that we didn’t really settle down into the game at all.
"You are going to face some pressure when you play away from home in Europe and weather the storm and get to grips with it, but we never really did so that was a disappointment."
On taking AZ back to London next for the second-leg, Postecoglou is confident his players can find a way past the Dutch defence.
He stated, "I don't have any doubt about our ability to break them down, but obviously we have to play a little bit differently to how we did tonight, not just with ball but without the ball too. We've got to be a lot more aggressive than we were tonight. We were just lacking a bit of aggression in everything we did.
"We were a little bit hesitant with the ball, hesitant in our pressing and we can't be that way. If we address those things I'm sure we'll be in a position to at least cause them more problems that we did tonight. It's only 1-0 so I guess that’s a positive in that we didn’t let the game get away from us."