Ed Griffiths agreed to lead Saracens for 12 months but leaves the Premiership club just weeks after embarking on his second spell in charge.
Bordeaux-Begles, nudged into second on Saturday, could not return to the Top 14 summit at Toulouse.
In their first Premiership outing since their automatic relegation was confirmed, Saracens were soundly beaten by London rivals Harlequins.
Top 14 front-runners Lyon won again, while La Rochelle's Arthur Retiere grabbed a hat-trick and Brive left it late.
Rhyno Smith scored twice in the closing stages against Southern Kings to again secure the bragging rights for the Cheetahs.
Exeter Chiefs rallied in the second half against Sale Sharks, but the Premiership leaders still slipped to a first defeat since November.
Northampton Saints looked to be heading top of the Premiership until London Irish sprung a shock following Api Ratuniyarawa's dismissal.
Jacques Nienaber had previously served as Rassie Erasmus' assistant with South Africa and has now taken the top job.
Trent Robinson says Michael Cheika is helping Sydney Roosters push themselves further ahead of the new NRL season.
Michael Fatialofa is showing "encouraging signs of improvement" and has been moved to the Major Trauma Unit at St Mary's Hospital.
Saracens owner Nigel Wray apologised for his "ill-considered approach" after a report detailed how Saracens fell foul of salary-cap rules.
The mercurial Finn Russell has been punished for a breach of team protocol just nine days before Scotland's first Six Nations game.
New Scotland captain Stuart Hogg proved if you don't ask, you don't get when he put himself forward to lead his country.
Saracens have given Premiership Rugby permission to release the full report into their salary cap breaches, which have led to relegation.
England captain Owen Farrell had the chance to confirm he would stay with crisis-hit Saracens, but instead he said talks were "ongoing".
France captain Charles Ollivon does not believe Saracens' impending relegation from the Premiership will have any impact on England.