Reduced to ten men, the Blues were immediately put under pressure and United capitalised.
The final blow came late in the game when Trevoh Chalobah headed in a goal from a corner inside the six-yard box.
The Gunners easily tore through Chelsea’s defence line, scoring 5 to set a new record against London rival.
It was a case of what might have been for Chelsea, whose winless run extends to seven matches across all competitions.
Brighton hammered Chelsea 4-1 at the Falmer Stadium to give The Blues their first loss since the appointment of Graham Potter in September.
A lone strike by defender Ben Chilwell saw Chelsea keep their grip atop the English Premier League despite Brentford’s late game push.
