Celtic qualified for the lucrative Champions League group stage on Tuesday despite losing 4-3 to Astana in the second leg of their play-off in Kazakhstan.
Patrick Twumasi scored twice as Astana, beaten 5-0 last week in Glasgow, threatened an incredible comeback before Brendan Rodgers' side secured their place with an 8-4 aggregate victory.
"I have mixed emotions because we didn't defend well enough," Rodgers told BT Sport after a scare that brought back memories of the same stage 12 months ago.
Celtic beat Hapoel Be'er Sheva of Israel 5-2 at home in the first leg of the play-offs last year and then wobbled in the return, losing 2-0 to scrape through.
"We had our Be'er Sheva moment. I thought we were going to get through this campaign without the anxiety. The players were on the ropes for 15 or 20 minutes but then they regained composure."
Kristoffer Ajer diverted a Dmitri Shomko shot beyond Craig Gordon as Astana took the lead on 26 minutes, but Scott Sinclair settled Celtic nerves with a superb equaliser.
That left Astana needing to score six more times to go through but Serikzhan Muzhikov drilled them back ahead on the night before Ghanaian striker Twumasi added a brace to leave Celtic rattled and facing a tense final 20 minutes.
However, Olivier Ntcham ended Astana's hopes when his deflected strike made it 4-2 before Leigh Griffiths reduced the deficit further in the final minute.