Chelsea lost their fifth successive game since the return of Frank Lampard as Brentford won at Stamford Bridge to strengthen their position in the Premier League's top 10.
The visitors did not have a shot on target in the first half but took the lead midway through when Mathias Jorgensen's flick from a corner hit Cesar Azpilicueta on his shoulder to deflect into the back of the net.
Chelsea once again struggled to create chances and a hugely disappointing first half ended with boos from some of the home supporters.
The Blues did improve after the break, thanks largely to the introduction of striker Pierre-Emerick Aubameyang, who mis-hit an overhead kick from a dangerous position before firing a low strike at Brentford goalkeeper David Raya.