Paris Saint-Germain captain Marquinhos and coach Mauricio Pochettino both slammed the performance of the Ligue 1 leaders in Sunday's defeat to Monaco.
The 3-0 loss, in which Wissam Ben Yedder scored a double either side of Kevin Volland's goal, is the heaviest defeat PSG have suffered in Ligue 1 since a 5-1 loss at Lille in April 2019.
PSG have now lost three of their last five games in Ligue 1, as many as they did from their previous 36 matches (27 wins and six draws).
It also represented PSG's heaviest loss to Monaco since September 1999, when the principality club defeated them by the same scoreline in Paris, and Marquinhos did not hold back in his criticism of the display.
"It's hard to digest. It's our worst game of the year," he told reporters.
"We came here to have fun and win a game. We are in a difficult moment.
"You can't just throw it all away. We all have to work together to get out of this moment."
Marquinhos stopped short of suggesting that PSG's recent Champions League elimination at Real Madrid has led the team to drop off, but claimed they could still lose their 13-point Ligue 1 lead without major improvements.
"It is not a relaxation," he added. "There were a lot of expectations [against Real Madrid]. It's a defeat that hurts us.
"Now we have goals to look for. We have to do our best, with the right mentality.
"If we continue to perform like that, it is sure that the title is in danger. The alert is there. We will have to come back [from the international break] with a good state of mind."
PSG boss Pochettino has now lost eight of his first 50 league games in the Parisian dugout, the same amount that his predecessor Thomas Tuchel suffered in his first half-century of matches.
Pochettino repeated Marquinhos's criticism of the performance after the match, calling his team's display "unacceptable".
"It is inadmissible to start this way," Pochettino said of PSG's dire first-half showing. "Afterwards, there is sporting shame.
"Now we have to move on, get over the frustration of the Champions League and think about giving our best, and have a competitive spirit.
"This is unacceptable. This first half is unacceptable. We didn't have a fighting spirit, the way we played is not acceptable.
"This cannot happen, we can have [bad] performances, but this type of attitude is not possible. It's unacceptable to start the match this way. Respect for the club, [and for] our supporters, is essential."