Thomas Tuchel condemned Paris Saint-Germain's attitude in their shock collapse to Monaco but could at least use the match to prepare for RB Leipzig, where Neymar will start.
PSG were cruising at half-time on their return to Ligue 1 action on Friday as the fit-again Kylian Mbappe scored twice.
But a third effort from the France international was ruled out and the half-time introduction of Cesc Fabregas changed the game.
Kevin Volland swiftly pulled a goal back for Monaco, before Fabregas teed up the German forward's second and then swept in the winner from the penalty spot after Abdou Diallo was sent off.
Fabregas created three chances - no player managed more despite the midfielder only playing 45 minutes - and also had three shots, trailing only Volland.
The former Chelsea midfielder completed 32 of his 36 passes, including 86.4 per cent of his attempts in the PSG half, but Tuchel said his side were to blame as they ceded control.
Diallo's costly foul was one of 12 from the visitors in the second half, while Monaco won five more duels than their opponents across the 90 minutes.
"We controlled the first half with two goals scored and two others [disallowed], I think the third was valid," PSG boss Tuchel told Telefoot after the 3-2 loss.
"In the second half, we lacked the same concentration, we did not play seriously enough, we did not take the ball with the same quality and we didn't defend seriously enough either.
"This is why we are completely responsible. Maybe it was too easy and the players got the impression that it was already 3-0 or 4-0 when it was only 2-0.
"We have to accept the criticism because we have completely stopped playing football.
"We have not done what is necessary to win a match: defending in a block, remaining aggressive, remaining attentive in behind.
"We have lost also a little strength in the midfield but we had to make changes. It's always possible to lose but losing a match like that is very weird.
"In the first two minutes of the second half, we had two very high recoveries but then we stopped completely. It was not possible to win the duels; we were always late. It is not normal."
It was the first time PSG had let a two-goal slip to lose in the league since January 2015, a 4-2 reverse against Bastia.
Tuchel acknowledged the champions were hamstrung a little by the looming home Champions League clash with Leipzig, though.
Neymar was returning from injury and had to appear from the bench, while Mbappe lasted only 68 minutes. Marquinhos did not play at all.
"The match against Leipzig is going to be very difficult, but it would not have been much different if we'd won," Tuchel said of Tuesday's continental clash.
"We had to get Kylian out, not take any risk with Marquinhos and make Neymar play for a few minutes.
"The mix between the two matches, Monaco and Leipzig, is perhaps also a reason which explains why we lost our concentration."
He added: "Neymar is determined. He will start against Leipzig."