PSG, who spent more than 80 million euros on transfers last summer, were humbled 3-2 by promoted Dijon in a League Cup last-16 tie on Wednesday after leading 2-0.
Playmaker Javier Pastore was rested for the game and second-half substitutes Kevin Gameiro and Nene, who have been in great form this season, failed to turn the game around.
"We must learn from these mistakes. We must be more focused, especially defensively," said coach Antoine Kombouare.
Last crowned champions in 1994, PSG gave away two penalties and Uruguay centre-back Diego Lugano looked a pale shadow of his international self.
"Maybe we were too self-satisfied. This game has shown that we cannot become lax if we are to stay at the top," said midfielder Christophe Jallet.
With main shareholders QSI (Qatari Sport Investment) looking ready to spend more millions on the transfer market, rumours have been flying around - jokes, too.
French daily L'Equipe reported on Wednesday that PSG would offer champions Lille 50 million euros for Belgian prodigy Eden Hazard in January as rumours of former England captain David Beckham also joining the club grew louder.
"I also heard they would buy Luchin (Lille's training ground) so that they can replace the Camp des Loges (PSG's training ground) with it," said Lille coach Rudi Garcia.
"And I also heard they are considering extending the Channel Tunnel to Paris."
Lille, who are third and four points off the pace, travel to neighbours Valenciennes on Sunday looking for their fourth consecutive Ligue 1 win.
Fifth-placed Olympique Lyon host Saint-Etienne on Saturday, three days after the League Cup game at the Geoffroy Guichard stadium, won by the visitors 2-1, turned sour.
The game was interrupted for five minutes as local fans threw flares on to the pitch after Lyon were awarded a penalty.
Michel Bastos converted it but picked up a yellow card after referee Clement Turpin considered the Brazil winger had provoked the Saint-Etienne supporters with his goal celebration.
Bastos denied wrongdoing, saying the gesture was intended for his wife.
"We will go for revenge on Saturday," said Saint-Etienne forward Lynel Kitambala. "This derby has always been an important game."
Lyon are expected still to be without Argentine striker Lisandro Lopez, who has been sidelined for two months with an ankle injury. "We miss him. He is like a guide to this team," said Lyon coach Remi Garde.
