Peter Bosz is on the brink of being sacked after his Borussia Dortmund side were beaten 2-1 at home by Bundesliga strugglers Werder Bremen on Saturday.
Defeat extends Dortmund's wait for a league win to eight matches and the under-pressure Dutchman will struggle to survive such a poor result after sporting director Michael Zorc indicated it was a must-win fixture.
Bremen began the day second-bottom, despite an upturn in form under new coach Florian Kohfeldt, but the visitors fully deserved their shock win at a freezing Signal Iduna Park.
Maximilian Eggestein, who celebrated his 21st birthday on Friday, gave Bremen the lead with a sumptuous strike, finding space on the right side of the penalty area to whip an unstoppable shot into Roman Burki's top-right corner.
Dortmund did not record a single shot on target until the second half but levelled after 56 minutes, Pierre-Emerick Aubameyang scrambling home from close range after Shinji Kagawa headed Marcel Schmelzer's cross back into the box.
But Dortmund were undone by a set-piece after 65 minutes, Theodor Gebre Selassie powering a header past Burki from Max Kruse's right-wing corner, leaving Bosz in danger of being dismissed a mere five months into his reign.
Kruse, making his 200th Bundesliga appearance, had the first clear sight of goal after 12 minutes but Burki collected his snapshot at the second attempt.
Zlatko Junuzovic then lashed over the angle as Bremen, who won away from home in the Bundesliga for the first time since April, continued to make the running.
And Bremen took a deserved lead after 25 minutes in which Dortmund had offered nothing.
Eggestein collected a pass from Kruse, cut inside off the right flank to beat Raphael Guerreiro and curled a stunning left-footed effort out of Burki's reach.
Aubameyang had a weak penalty appeal rejected after he was bumped over in the box by Philipp Bargfrede and Andriy Yarmolenko sent a tame effort wide after 40 minutes but the half ended with visiting goalkeeper Jiri Pavlenka having been completely untested.
Bosz, sensing the need to make radical changes to save his job, introduced Andre Schurrle and Nuri Sahin for Yarmolenko and Guerreiro, who had struggled in a central midfield role, at half-time..
Those switches yielded an immediate Dortmund improvement and Kagawa stung the palms of Pavlenka with a ferocious hit.
But Pavlenka could do little about Dortmund's equaliser, Aubameyang on target from point-blank range after Schmelzer's cross was cleverly headed back into the danger zone by Kagawa.
With Dortmund pushing to take the lead, Bremen wasted a terrific chance on the break break with four men on two, then Burki made a brilliant save down to his left to deny Kruse, who had scored four goals in his last three Bundesliga games.
Burki was helpless as Bremen nudged back in front from from the resulting corner, however, Gebre Selassie getting up highest to thump in a terrific header.
Kagawa should have levelled after 71 minutes but thumped his close-range shot into a prone Aubameyang, the ball looping over the crossbar to sum up Bosz's seemingly ill-fated reign, then Schurrle fired straight at Pavlenka.
Bosz sent for 18-year-old striker Alexander Isak with his last throw of the dice, but Bremen held on for their first win at Dortmund since April 2007 to leave the Dutch coach in serious trouble.
Key Opta stats:
- Borussia Dortmund have won none of their last eight Bundesliga games (three draws, five losses) – their longest winless run in more than 17 years (February to April 2000, 14 matches).
- Dortmund have conceded 12 goals in their last four Bundesliga home games – as many as in the entire 2016-17 season in 17 home games.