Graham Potter suffered defeat for the first time as Chelsea boss, losing 4-1 to his former employers Brighton & Hove Albion at the Amex Stadium.
The hosts wasted absolutely no time in taking the game to Chelsea, as Leandro Trossard lobbed Kepa Arrizabalaga, only for Thiago Silva to head the ball clear from the goal line.
The Blues failed to heed that early warning and were behind within five minutes after a high press allowed Kaoru Mitoma to slip in Trossard, whose close control took him past Kepa to squeeze in the opener.
Shell-shocked, Chelsea searched for a response but a second own-goal on the stroke of HT all but sealed the points for the Seagulls.
Pervis Estupiñán raced down the left wing before squaring, persuading Trevoh Chalobah to slide in and unwittingly turn the ball past Kepa.
Conor Gallagher whipped in a dangerous ball to the far post after good work from Loftus-Cheek, leaving Havertz to head home and reignite Chelsea hope.
As for Chelsea, the demoralising defeat leaves them fifth in the Premier League, just three points ahead of Brighton in seventh.