Chelsea crumbled to a damaging 5-2 defeat at PSG in the first leg of their Champions League last-16 tie.
The Blues matched the current Champions League holders for much of the game and twice came from behind to level.
But a goalkeeping howler from Filip Jorgensen allowed Vitinha to score in the 74th minute and sparked a spectacular collapse from Liam Rosenior's side, giving them with a mountain to climb in next week's second leg at Stamford Bridge.
"We didn't react well to a setback," the Chelsea boss told TNT Sports. "At 3-2 we are in the tie. Then Kvaratskhelia scores an unbelievable strike and that happens. At this level, there are quality of players on both sides.
"The fifth goal is after a flashpoint. We don't keep our heads in that moment, we don't keep our cool. They break our press and score a fifth goal. It makes it very difficult to come back from but it's not impossible."
PSG were under pressure after being beaten by Monaco on Friday, with some of their players under scrutiny for not performing at their best this season. One of those was Bradley Barcola, who answered his critics by smashing home the opener via the underside of the crossbar.
Chelsea deserved to equalise though - despite lacking a finishing touch at times - and made the breakthrough just before the half-hour. Barcola was caught napping as an unmarked Malo Gusto's shot bobbled goalwards, Matvei Safonov getting a hand on the ball as it bounced past him and in.
PSG retook the lead five minutes before half-time. Fourteen seconds after Cole Palmer saw a header saved by the goalkeeper, the hosts hit Chelsea on the break. Ousmane Dembele bamboozled Wesley Fofana at the top of the box before firing past Jorgensen.
A quiet start to the second half soon sprang into life as Enzo Fernandez hauled Chelsea level again. Pedro Neto stormed down the left-hand side before finding his team-mate, with the Argentine's confident finish arrowing into the back of the net.
But Jorgensen undid all of the good work with a horrendous mistake for PSG's third. His attempted pass out from the back was intercepted by Barcola, who in turn found Khvicha Kvaratskhelia. He fed Vitinha who chipped over the devastated goalkeeper.
At that point, it still looked like Chelsea would be in with a chance of a result in the second leg - but two fantastic strikes from Kvaratskhelia secured what looks to be an unassailable lead for PSG.
The first was a stunning hit from distance, before slotting home the fifth from close range with the final touch of the game. It came after another PSG goal had been ruled for offside, which again came from a Jorgensen error - a let-off for Chelsea.
The game boiled over towards the end after Neto pushed a ball boy after not being given a ball as quick as he would have liked. Players from each side were involved in a melee as VAR checked for a red card, but ultimately nothing was given.
The result will go some way as PSG seek revenge for a 3-0 defeat by Chelsea in the Club World Cup final in July. The Parisians will not play in Ligue 1 this weekend as they are given every chance to prepare for next Tuesday's second leg, while Chelsea face Newcastle in the Premier League on Saturday, live on Your Site.
Chelsea boss Liam Rosenior to TNT Sports on Filip Jorgensen: "You have risks playing every way you play. You can kick long and it goes straight back into your net. You can build. That's the risk that we take. I take responsibility. I'm accountable for that.
"It's a shame because, because of those build moments we have more control of the game. Somehow, they've had 0.8 xG and have scored five. That's something really really difficult to swallow. We don't have a lot of time to digest. We have to get back on the plane, get ready for Newcastle and then PSG next week."
"That's on me [to deal with confidence]. You always lose confidence with mistakes but we have a really good group of lads. We don't blame for mistakes, but we don't make the same mistakes twice."
Analysis from Your Site' Charlotte Marsh:
Many said 'English football is back on top' after five of the six Premier League teams finished in the league phase top eight, giving them a direct route to the last-16.
But that theory has come undone this week, with none of the teams winning their fixtures. Chelsea and Spurs were beaten heavily, as were Man City. Liverpool were also beaten by Galatasaray while Arsenal had to come from behind to draw in Leverkusen. Newcastle suffered a heartbreaking late draw against Barcelona too.
The good news is that for all of the sides bar Newcastle, they have home games to look forward to next week and will need to harness that power. As football has taught us many times before, it's not over until it's over.
However, another worrying trend has been goalkeeping errors, especially for a different player coming in from the previous game.
Antonin Kinsky's well-documented 17-minute cameo ended in disaster for Spurs after he replaced Guglielmo Vicario. You wonder if his career will ever recover.
Then, Filip Jorgensen replaced Robert Sanchez for Chelsea on Wednesday. This was a more obvious and expected change, with both experienced goalkeepers in their own right.
But Jorgensen looked uncomfortable at times with the ball at his feet. It was one of these moments that saw Vitinha eventually scored PSG's fifth - and that's where Chelsea's game crumbled.
The Premier League is still the best and most exciting league in the world, and remains on course for another five teams in the Champions League next season.
But there will surely be question marks over the quality and competitiveness in Europe after this week's showings, and whether that fifth place is fully justified.
Chelsea winger Pedro Neto to TNT Sports: "I want to say the situation that happened on the pitch with the ball boy, I want to apologise. I already spoke with him. It was the heat of the moment. We were losing and I wanted to pick up the ball. I gave him a little push and I saw that I hurt him. I am not like this.
"I'm really sorry about that. I'm sorry to him... He came to speak with me and he laughed because I gave him my shirt. I said sorry about 35 times."
Chelsea boss Liam Rosenior added on the incident: "I haven't seen what happened. I heard there was an altercation with a ball boy and you never want to see that. I've been told Pedro has apologised. I have to see it back and speak on it in the days to come.
"In that moment, as a group - that's happened. Switch on. We didn't and we were punished by a good team."
Round of 16 second legs: March 17-18
Quarter-finals: April 7-8 and 14-15
Semi-finals: April 28-29 and May 5-6
Final: May 30 (Puskas Arena in Budapest, Hungary)
