Thomas Tuchel's side were stunned by rock-bottom Darmstadt on Saturday and the forward says his team put in an "unacceptable" display
Marco Reus slammed Borussia Dortmund's woeful defending in their shock 2-1 defeat to Bundesliga relegation battlers Darmstadt.
Terrence Boyd and Antonio Colak scored either side of Raphael Guerreiro's strike to condemn Thomas Tuchel's side to defeat, marking Torsten Frings' first win in charge of Darmstadt.
Dortmund will slip out of the top four if Hoffenheim avoid defeat against Wolfsburg on Sunday and Reus was angry about his side's lacklustre display.
"We simply didn't play well," he said in quotes reported on Dortmund's website.