WATCH: Mbappe miss proves costly as Rennes stuns PSG
PSG began the year with a seven-point lead at the top of Ligue 1, but that has been clipped to three after defeats to Lens and now Rennes, which beat the Parisians 1-0 on Monday.
Head coach Galtier will face scrutiny for such domestic losses, given the superstars at his disposal, and he knows he and the team must start to show their best again.
Galtier said: "I'm mainly disappointed with our performance. We didn't create anything, we had very few favourable situations.
"We focused on having possession of the ball and we had far too many players who dropped back without having players high enough to secure the Rennes defensive line."
PSG had only eight attempts on goal, their lowest total in a league game this season, and only one went on target. The defeat was a third in their last four trips to Rennes in the league, with the Breton side firmly established as a bogey side for the capital giants.
Galtier pointed to issues with Lionel Messi and Neymar having no obvious targets when looking to move the ball forward, and that situation persisted when Kylian Mbappe came on as a substitute early in the second half, replacing the ineffective Hugo Etikike.
The former Lille, Saint-Etienne and Nice coach said there was an "urgent" need for PSG to get back on track.
"Obviously we're going to have to find a lot more play and connection between the lines," he said. "We knew we would have a difficult match, and we had it. The opposing goalkeeper was barely troubled. We need to find a lot more rhythm, intensity and technical relationship between one another.
"I'm not going to talk about worry, but there has to be awareness. We can find a thousand excuses, but the World Cup is over.
"Even if we have been dispersed for eight weeks, we need to find cohesion and rhythm in our game."
PSG will head to Saudi Arabia next to face a Riyadh Season Team, made up of players from Al Hilal and Al Nassr, in an exhibition game on Friday (AEDT).
Al Nassr's new signing Cristiano Ronaldo could be involved in that game, but Galtier wants intense focus from his players when they get a chance to train on the getaway.
They do not have another Ligue 1 game until Reims visit the Parc des Princes on January 29, although there is a Coupe de France tie against minnows Pays de Cassel on January 23.
"We will have 10 days to work," Galtier said.
Lens are their nearest challengers, but Marseille are only five points adrift of PSG.
"The championship is tight," Galtier said. "I'm not used to looking at the standings but I'm concentrating on performance and I have work to do."
Warren Zaire-Emery, at the age of 16 years and 313 days, became the youngest player in PSG's history to start a Ligue 1 match. He played 72 minutes before being replaced by Carlos Soler.
Galtier will gather his players to show them video comparisons with how the team are performing now and how they played prior to the World Cup.
Mbappe was the World Cup's top scorer, while Messi brilliantly captained Argentina to the trophy, being named player of the tournament. Such form has been mostly lacking since resuming PSG duties, but they are not the only ones culpable.
Goalkeeper Gianluigi Donnarumma, who was absent from Qatar 2022 after Italy failed to qualify, said PSG's players are furious by their collective drop-off in performance.
"We are angry, and we have to bounce back immediately after this match, from this defeat, because we have no more time to lose," Donnarumma said.
"It was an avoidable loss. They didn't have many chances but we should have done more. We have to learn from this game. We are upset, but now we have to focus on the next game."