Marco Reus delivered a damning indictment of a "completely stupid" Borussia Dortmund performance following Saturday's humbling 3-1 defeat at the hands of promoted Union Berlin.
Marius Bulter's brace guided the capital club to their maiden Bundesliga victory and ended Dortmund's perfect start to the season.
Last season's runners-up appeared to have atoned for a poor start when Paco Alcacer cancelled out Bulter's opener but the Union winger added a second after the interval before Sebastian Andersson sealed the upset in the 75th minute.
Lucien Favre blamed a lack of patience for his team's demise but captain Reus was far more scathing.
"We just played a completely stupid game," Reus told Sky Sport Germany.
"We had some good moments at times but were too sloppy in the final third.
"Sometimes we think we can win these games easily with our quality. That has to stop. We have to show the virtues that made us strong last season.
"That means desire and passion, and then you can have fun. Each one of us has to do a lot more."
There was a glimmer of good news for Dortmund on Saturday as sporting director Michael Zorc suggested Raphael Guerreiro could stay beyond the end of Monday's European transfer deadline.
The Portugal left-back, who featured as a substitute against Union, has been linked to Paris Saint-Germain and is out of contract at the end of the season.
"We are currently in talks and I would be pleased if he extends," Zorc said.
"I assume, as of now, that he will stay with us."
Former Dortmund defender Neven Subotic played the full 90 minutes for Union after joining over the summer, and was warmly received by the travelling Dortmund fans.
"I wanted to express my respect, even in a moment of joy - this appreciation is very important for me and for the fans," he said after the game.
"It was also beautiful after the game, to greet the fans, and to say thank you again for the years of support, so I tried to keep things under control."
Dortmund will look to bounce back with two home games after the international break. They welcome Bayer Leverkusen in the Bundesliga before a mouthwatering Champions League clash with Barcelona.