Thomas Tuchel's side, depleted by a raft of injuries, slumped to a 2-0 loss against Reims on Wednesday which ended an unbeaten 22-match run at the Parc des Princes.
Discounting a handful of defeats after winning the league in previous seasons, it was PSG's first meaningful loss at home since November 2012.
"It was a poor performance. We had a lot of players missing, that's why we made so many changes," said Tuchel.
'We deserved to lose': Injury-hit PSG slump to shock home defeat by Reims
"I have to manage the playing time of the squad. And I think it was possible to play better with the team we had on the pitch."
PSG remain top of Ligue 1 on goal difference, above surprise second-place side Angers, but defender Marquinhos said they must strive to consistently keep the bar high.
"The challenge for this season will be to maintain a high level each match," said Marquinhos. "We must inspire ourselves to rise to this challenge."
Neymar, who produced late match-winning strikes in his first two games this season, cut a peripheral figure midweek with Mbappe, Edinson Cavani and Mauro Icardi all missing.
Tuchel spoke of taking a cautious approach with the 20-year-old Mbappe, who has missed the last five matches but is back in training with the squad.
Lyon's bright start under first-time manager Sylvinho is a distant memory with the club winless in five matches since beginning the campaign with two resounding wins.
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The latest setback came at Brest where the hosts twice recovered to earn a 2-2 draw, a result that dropped Lyon into the bottom half of the table.
"We didn't play well. I'm not satisfied, once again we lost the three points in the final minutes," said Sylvinho, whose team host fourth-placed Nantes on Saturday.
"We have a lack of confidence and it generates fear. I am a little concerned about the results, but the first step is to restore confidence. To do that, we need to win matches."
Monaco picked up their first victory this term on Tuesday and will host Brest at the weekend, while Marseille -- held to a 0-0 draw at basement side Dijon -- are at home to Rennes on Sunday.
While Nicolas Pepe is slowly adjusting to life in the Premier League at Arsenal, Victor Osimhen has made an electric start in Lille colours after arriving from Belgian club Charleroi.
The 20-year-old Nigerian was limited to a 45-minute appearance in the third-place play-off match at the Africa Cup of Nations but has scored six times in his first seven Ligue 1 games for Lille.
He is the joint top-scorer in the division alongside Moussa Dembele and will attempt to inflict more damage this weekend on Nice, who are reeling from Tuesday's 3-1 loss at struggling Cote d'Azur rivals Monaco.
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