The fifth-round encounter was tightly contested during the opening 45 minutes, with Chelsea enjoying spells of possession and creating openings around the Brentford penalty area. However, the visitors lacked the finishing touch needed to convert their opportunities as both sides went into the break level.
The game changed shortly after the restart when Brentford finally found a way through in the 61st minute.
Jannik Schuster delivered a long ball into the Chelsea area, and Jaidon Anthony reacted well to head home from close range and give the hosts a 1-0 advantage.
Chelsea immediately looked for a response, with Pedro Neto, Morgan Rogers and Cole Palmer involved in a series of attacking moves. Neto saw a promising effort blocked, while Palmer also had a shot from inside the box stopped as Chelsea attempted to restore parity.
Brentford, however, grew increasingly confident as Chelsea committed more players forward.
The hosts doubled their lead in the 83rd minute when Igor Thiago reacted quickest to a rebound inside the penalty area. The forward kept his composure to fire the loose ball into the right side of the net, leaving Chelsea with a mountain to climb.
Chelsea pushed forward in search of a lifeline, but their increased attacking commitment left spaces for Brentford to exploit.
The home side completed the rout in the fourth minute of stoppage time. Kevin Schade helped create the opening during a scramble inside the Chelsea box before Fabio Carvalho calmly finished into the right side of the goal to make it 3-0.
The emphatic result extended Chelsea’s difficult spell in the Premier League, with Alonso’s team now having gone three consecutive league matches without a victory.
For Brentford, the win was a reward for a disciplined performance that combined defensive organisation with clinical finishing after half-time. The hosts absorbed Chelsea’s early pressure before making their opportunities count when the match opened up.
Chelsea, meanwhile, were unable to recover after conceding the opener. Their attempts to find a way back became increasingly urgent, but Brentford’s defence repeatedly stood firm while the hosts remained a threat on the break.
The defeat leaves Alonso and his players facing renewed questions over their consistency as the Premier League campaign gathers pace, while Brentford can take confidence from a comprehensive victory built on resilience, efficiency and a powerful second-half showing.