Marco Reus said Borussia Dortmund's performance against Darmstadt was "not acceptable" as his side suffered a 2-1 defeat away to the Bundesliga's bottom team.
Antonio Colak's 67th-minute goal put Darmstadt ahead for good, and Reus was particularly upset with Dortmund's defending on the night.
"We simply didn't play well," he told Dortmund's website. "All of the players defended disastrously.
"Darmstadt had a real lot of chances and they were only playing with long balls. In the second half we really didn't play well at all, we can have no complaints. Unfortunately the defeat is a fair result.
"We didn't play the game that the coach wanted us to. We didn't dare to make the passes. The manner of the defeat was not acceptable. We need to talk about what happened, we can't just let it go.
"Yet again we have failed to win against an opponent that we really should have beaten. And that's really bad."
The result allowed leaders Bayern Munich to increase their lead over Dortmund to a nearly insurmountable 15 points, while Thomas Tuchel's men now find themselves in the thick of a fight just to qualify for the Champions League.
"It was a deserved victory for them," Tuchel said of Darmstadt. "If we play so far below our limit, it is normal to lose the game.
"It makes sense that the team which plays to its full potential wins. We were deserved losers today. I never had the feeling today that we ever really got into the game."
Darmstadt boss Torsten Frings was happy for new signing Hamit Altintop, whose addition was not widely welcomed by the club's supporters.
"All the players were outstanding today," Frings said. "And some people should apologise to Hamit Altintop.
"Even when it was 1-1 we wanted to play and be brave. I'm incredibly proud of the troops. It's great that the team has finally rewarded."