Mauricio Pochettino says the "next few weeks are not going to be easy" after his Paris St-Germain side let a 2-0 aggregate lead slip in their Champions League exit to Real Madrid.
With an hour gone in the second leg, the visitors were in complete control thanks to Kylian Mbappe's second goal of the tie.
But they were left stunned by Karim Benzema's 17-minute hat-trick and suffered another painful exit from the competition they covet more than any other.
"The first [Benzema] goal completely changed the game," said Pochettino.
"For an hour, we were better than Real Madrid. The atmosphere changed in the stadium. We made some mistakes after that, we can't say we didn't. The worst feeling is that we were the better side, but we lost the tie in 10 minutes.
"Paris St-Germain have been chasing the Champions League for a number of years now.
"I'm really disappointed, upset - but these things can happen. The next few weeks are not going to be easy."
Since moving under Qatari ownership in 2011, PSG have assembled a series of star-studded squads.
But their dreams of winning European club football's elite competition have been repeatedly undermined by several collapses from winning positions, including the 6-1 thrashing by Barcelona in 2017 and the last-16 defeat by Manchester United in 2019.
"We have been here before with PSG. Too many times, this is a huge failure. A lot is down to the players on the pitch because you can't make these mistakes," French football journalist Julien Laurens told BBC Radio 5 Live.
"Pochettino will not be there next season, I can tell you that - unless they can't get [Zinedine] Zidane in. I don't even think he wants to be there; he wants to be in England.
"His family is still in London and he has been in a hotel in Paris for a year and maybe that is part of the problem. You can't let your team sink like they did tonight without responding. He has been too reactive."
Laurens also criticised Pochettino's regular rotating of his two goalkeepers, Gianluigi Donnarumma and Keylor Navas, and questioned whether the players had the right focus to improve their away form - they have won just one of four Ligue 1 matches on their travels since the turn of the year.
"The goalkeeping situation is crazy," Laurens said. "He doesn't put them in the right environment to be good. You never know who will play next.
"The away form is not good enough - not just in the Champions League. They have a huge problem psychologically away from home because they don't put the effort in. They can't grind out a result when under pressure.
"The complacency is what gets me - when you have thrown away leads like this and yet you are still complacent like they were tonight. This is the Champions League and it came back to haunt them."
'It is part of PSG's ingrained culture'
Former England midfielder Steve McManaman, who won the Champions League twice during his time at Real Madrid, gave a damning assessment of another PSG failure in the competition.
The Ligue 1 leaders reached the final and semi-finals in consecutive seasons but have also been knocked out at the last-16 stage in four of their past six Champions League campaigns.
"We expected PSG to unravel. They did what they always do, when they come under pressure, they melt away," said McManaman on BT Sport.
"There was no organisation and no leadership. The players are too powerful. When the going gets tough, they do not care. They just play whatever they want to play.
"You can't have four or five [players] not fancy it when the going gets tough.
"PSG made mistakes constantly in the second half. You have to give Real credit because even when they are down, they stay in the game.
"We have seen managers come and go and they cannot sort it out. They come, they get knocked out of the Champions League and get sacked and then move on."
Former Manchester United defender Rio Ferdinand echoed those sentiments, adding: "It is the mentality and maybe the culture of the club.
"This isn't something that is new at the club. They have fantastic talent in the squad but when the mentality isn't right on the day, you have to stay in the game. They didn't do that. It is part of the ingrained culture at PSG."