It was the third European competition Celtic have been knocked out of this season, following defeats in the Champions League and Europa League.

"When you look at our European campaign, ultimately we haven't succeeded," said Postecoglou.

"That to me is the bottom line, irrespective of progress or improvement or learnings.

"This football club should be making an impact in Europe and it's my job to make sure we do."

Hopes of making the last 16 were extinguished after just nine minutes when Ola Solbakken swept home for the hosts.

And the Scottish Premiership leaders were undone again as Hugo Vetlesen knocked in a second.

"It's disappointing for us," said Postecoglou, whose side have lost seven of their 14 European games. "We certainly wanted to make progress in this competition, but unfortunately our performances haven't warranted it.

"If you don't perform at the levels we have been, then we won't be successful.

"They were probably our least effective two performances in Europe this year, including when we got beaten 4-0 by Leverkusen.

"When you are at that level you don't get the opportunity to progress."

Postecoglou's side fell at the first hurdle in Champions League qualifying when they were defeated by Midtjylland, and finished third in their Europa League group.

"I expect to go further in every competition we play, that's the nature of this football club," he said.

"Europe for us was beyond us in terms of levels we needed to get to. It's my responsibility to make sure we improve and next year ensure that we perform better.

"I think the supporters understand where we're at and what we need to do."