Bournemouth - Maurizio Sarri's first season at Chelsea descended into crisis as his spluttering side slumped to an embarrassing 4-0 defeat at Bournemouth on Wednesday to drop to fifth in the Premier League.
Two second-half goals from Joshua King, either side of a well-taken effort by David Brooks, and Charlie Daniels's late header inflicted a third successive away defeat on Sarri's team.
The Blues have now lost five Premier League games, including their last two fixtures, in just over two months, a wretched run that has cost them their place in the top four.
Arsenal move above Chelsea, who are level on points with their London rivals but drop down to fifth on goals scored.
Chelsea lost a Premier League game by a four-goal margin for the first time since 1996.
With fast-improving Manchester United only two points behind, this abject performance demonstrated the need for Sarri to inject fresh impetus into Chelsea's inconsistent campaign.
Adding to his woes, Chelsea fans chanted "you don't know what you're doing" at Sarri after he took off new loan signing Gonzalo Higuain in the second half.
The striker, who worked with Sarri at Napoli, was brought in to offer the focal point up front that Alvaro Morata and Olivier Giroud had failed to provide.
But against Bournemouth, Higuain, along with the rest of his team-mates, failed to deliver the cutting edge required to turn extended periods of possession into goals.
Sarri, who left Bayern Munich target Callum Hudson-Odoi out of the matchday squad, can ill afford many more performances like this one if Chelsea are to secure a return to the Champions League.
Bournemouth were without Callum Wilson, sidelined for three weeks after undergoing surgery on a knee problem and initially Eddie Howe's reshaped side struggled to get a foothold in the game.
Chelsea should have forced a seventh-minute lead when Pedro's cross from the right flew in front of Higuain before being met by Mateo Kovacic, whose header was well saved by Artur Boruc.
Sarri's side dominated much of the first half and spent long periods camped out in the Bournemouth half.
Yet for all their possession and territorial dominance, the visitors created only a limited number of chances and were guilty of over-elaborating on the fringes of the Bournemouth penalty area.
After Kovacic's early effort, Pedro created an opening after cutting in from the right to draw a save from Boruc.
Chelsea were handed a warning their failure to create more meaningful opportunities might prove costly towards the end of the first half.
Brooks and Junior Stanislas both forced Chelsea keeper Kepa Arrizabalaga into action with powerful shots that capped pacy breakaway moves.
Brooks had spurned an earlier chance when he failed to control a ball inside from Ryan Fraser but he showed much more composure when the winger clipped the ball into his path two minutes after the restart.
The Wales international looked up and pulled the ball back for King who drilled past Kepa.
Chelsea's response was limited and mirrored their efforts in the first half.
And it was Brooks's reading of David Luiz's misplaced pass that led to Bournemouth's 63rd-minute second.
The forward broke through Luiz's attempted recovering tackle before finding King, collecting the return and finishing well after rounding the defender.
King added his second in the 74th minute after being set up by Stanislas and Daniels headed home at the death.