Neil Lennon described Celtic's Europa League exit at the hands of FC Copenhagen on Thursday night as a lost opportunity for the Parkhead club.
The last-32 tie was finely balanced at 1-1 following the first-leg at the Parken stadium last week but the Danish side took the lead at Parkhead in the 51st minute through substitute Michael Santos when he took advantage of a slack back pass by Hoops defender Jozo Simunovic.
Striker Odsonne Edouard levelled with a VAR-awarded penalty in the 82nd minute and it looked like the tie could go to extra-time.
However, three minutes later midfielder Pep Biel restored the visitors' lead before striker Dame N'Doye, who scored last week, added a third in the 87th minute to seal a 4-2 aggregate defeat.
It was Celtic's first defeat of 2020 and a frustrated Hoops boss said: "It was an opportunity lost.
"Copenhagen didn't have to work hard to get goals, that was so frustrating and not like us at all.
"I am hugely disappointed. We shot ourselves in the foot.
"We had total control of game in first half, I don't remember Fraser Forster having anything to do of note.
"We hit the post and I was pretty pleased at half time and then we made mistakes and gave them oxygen and encouragement.
"We had to change the shape to get back into the game and deservedly got back in but we didn't manage the game, we shot ourselves in foot for second goal and that was the big moment of the tie.
"Individual mistakes cost us and that's the nuts and bolts of it.
"It was a great opportunity and we let it go. It was bitterly disappointing in the manner we lost."
FC Copenhagen would return to Glasgow if they are drawn against Rangers in the last 16 and boss Stale Solbakken said: "We had some players who are trying to come back from injury, train at Rangers' ground and they met Steven Gerrard and Gerrard obviously hoped that maybe we could meet in this tournament.
"Maybe that can happen.
"I seen Rangers play against Celtic in the League Cup final and in the league and it is two very even teams.
"That will be as hard and as tough and maybe even tougher because it is one more round now."