Neil Lennon accused Celtic of conceding "crazy, suicidal" goals against CFR Cluj as they suffered a shock Champions League qualifying defeat.
The Scottish champions were beaten 4-3 in Glasgow on Tuesday to lose the tie 5-4 on aggregate and miss out on a play-off match against Slavia Prague.
Celtic were 1-0 down at half-time but moved 2-1 up through James Forrest and Odsonne Edouard, only for Abdel Billel Omrani to level the scores from the penalty spot.
The hosts hit back through Ryan Christie but Omrani struck again to make it 3-3 before George Tucudean made Cluj's passage to the next round secure with the last kick of the game.
Celtic manager Lennon admitted his team were not good enough in the first half and felt they threw the lead away with their poor defending.
"The goals we conceded are crazy," he told BBC Radio Scotland. "You've got to do the basics well – you've got to stop crosses, close people down, get your head on things – and it was almost as if we scored and wanted to switch off and control the game.
"In the first half, we were way too passive, way too slow with the tempo, so we gave them a bit of a razz at half-time and we got the right response going 2-1 up. But we needed that two-goal cushion. It's suicidal the goals we gave away.
"The first goal, we got a warning before that, and we still didn't stop the cross and the guy's got across for a run. If you don't do the basics defensively well enough then, at this level, you get punished.
"We've let it go. We've only ourselves to blame. We had the lead and we let it slip through our own decision-making, really."
Lennon is keen to lift his side's spirits before they now prepare for a play-off to reach the Europa League group stages.
"We've got to work with the players. We've got to raise their morale, first of all," he said.
"We're going to get a bit of criticism and that's totally par for the course. We've worked really hard to get where we are and we've basically thrown it away. We've got some good defenders but for some reason they wanted to slow the game down."