Borussia Dortmund's Bundesliga title hopes were left in tatters on Sunday as they crashed to a 5-2 defeat at home to Bayer Leverkusen, with injured star striker Erling Haaland forced to watch from the sidelines.
In a game they had to win to keep pace in the title race, Dortmund's calamitous defending saw them slump to a sixth defeat of the season and fall further behind league leaders Bayern Munich.
"It was a catastrophic day for us. It's not the time to talk about Bayern, we have other issues at the moment," captain Marco Reus told DAZN.
They are now nine points adrift of the top, with their lead over third-placed Leverkusen slashed to just five points.
"It's a bitter defeat. Talking is one thing, but we have to put things into practice," said coach Marco Rose, who had promised a more solid defensive performance before the game.
Much had been made of Haaland's absence, but it was fellow convalescent Mats Hummels who was more sorely missed, as Dortmund's back line self-immolated in the first half.
Hummels' replacement, Dan-Axel Zagadou, handed Leverkusen the opener on a plate after just 10 minutes in a comedy of defensive errors.
Too hesitant on the ball after a short goal-kick, Zagadou gave the ball away cheaply in his own box, allowing Patrik Schick to shoot unchallenged from point-blank range.
As goalkeeper Gregor Kobel dived full-length to parry the shot, Manuel Akanji hurried back to help, only to end up bundling the ball into his own net.
Another own goal at the opposite end brought Dortmund level a few minutes later, Jeremie Frimpong flicking a Julian Brandt free-kick past his own goalkeeper.
But Leverkusen had smelt blood, and took the lead again with a devastating counter-attack, the entire front line linking up to provide Florian Wirtz with a tap-in.
Leverkusen midfielder Robert Andrich crowned a disastrous first half-hour for Dortmund when he curled a direct free-kick into the top corner from the edge of the area.
HALF-TIME BOOS
Booed off at halftime by their own fans, Dortmund briefly rallied at the start of the second half, before going 4-1 down on 53 minutes.
Having failed to properly clear a Leverkusen corner, the home side watched helplessly as Jonathan Tah sent a fantastic volley fizzing under the crossbar.
The return of US international Giovanni Reyna after six months out with injury provided some brief cheer on the hour mark, and both he and Jude Bellingham came close to scoring in the last quarter of an hour.
Leverkusen twisted the knife when Diaby netted a fifth on the counter-attack a few minutes from time, before Steffen Tigges grabbed a consolation tap-in.
In Sunday's other game, Wolfsburg hauled themselves away from the relegation zone with a 3-1 win over bottom club Greuther Fuerth.
Branimir Hrgota gave Fuerth hope from the penalty spot after Aster Vranckx had put Wolfsburg in front with an early tap-in.
Vranckx then restored the lead after a mistake from Fuerth goalkeeper Sascha Burchert, before Maximilian Arnold made it 3-1 with a curling free-kick.
Maximilian Philipp added a fourth with a deflected volley a few minutes later to seal a first league win in 10 games for Wolfsburg.