At one point in the second half, with the League One hosts leading 2-1, Brentford's raucous fans inquired of their visitors: "Champions of Europe? You're having a laugh." Chelsea did not even look like FA Cup holders for long periods.
They recovered to force a replay through Fernando Torres seven minutes from time but this was a deeply unconvincing performance.
Oscar scored an exquisite goal, Juan Mata brought intelligence when arriving for the second half, but too many high-profile names lost their duels with Uwe Rosler's players.
The replay will be on fifth-round weekend because of league, international and Europa League commitments. Chelsea fly back from Sparta Prague after their Valentine's Day assignment, resuming conflict with Brentford on February 17, before taking on Sparta in the second leg four days later.
They need Rafael Benitez to consider pairing Torres with Demba Ba, and urgently require Petr Cech to recover from what the interim manager termed "a niggle". Ross Turnbull is too underwhelming an understudy.
They also need Benitez to demonstrate more fully the tactical nous he was known for at Liverpool.
Those Chelsea fans temporarily relocated from the Shed to the Wendy House watched closely for signs that Benitez could influence the flow of the game. The criticism of Chelsea's interim manager from some supporters is that he responds to the clock rather than the scoreboard. They even chanted "You don't know what you're doing" when he delayed the introduction of Ba.
Benitez must shake off the impression that subs come on as if at pre-planned points in the game.
He had to act at the break, following a largely lifeless 45 minutes by Chelsea. Brentford were sharper, stronger of spirit, a collective rather than a gathering of individuals.
The pitch was poor in one half, part Hackney Marshes, part Copacabana, but Brentford managed to move the ball across it well enough. Adam Forshaw and Harry Forrester worked overtime. So did Jonathan Douglas, who wore the beard of a pirate and a look of utter determination to thwart distinguished guests. Marcello Trotta, on loan from Fulham, ran hard as the lone striker.
Chelsea knew swiftly they were in for a sustained examination of their character. With Eden Hazard starting his suspension, Chelsea emerged to a ballboy guard of honour, carrying ironic if unintended echoes of the Liberty Stadium.
They lacked inspiration and conviction in the opening half. Frank Lampard looked tired and, with the visitors' defence disorganised, Brentford grew in self-belief.
Two minutes from time, Turnbull erred again, parrying Forrester's shot into the path of Trotta, who pulled the trigger and three sides of Griffin Park rocked with delight.
One stand, housing the visitors, contained a mix of apprehension, defiance and soon dissent. At the break, Benitez walked across the pitch, his head down, initially engaged in conversation with Mata.
Benitez did not look to his right where a few Chelsea fans were giving him the reverse Churchill.
How would he react? Criticised for his delayed and ultimately ineffective substitutions against Swansea, Benitez actually twisted early. He withdrew Marko Marin, a skilful player who possesses the slightness of frame that would make kite-flying a risk in windy conditions yesterday let alone finding a way through the dense thicket of Brentford's five-man midfield.
If Mata's change brought some rare approving nods from the Chelsea fans, Benitez's decision to continue to keep Ba in reserve with Torres labouring was baffling. Torres needed support.
Oscar appreciated Mata's presence, the Brazilian equalising within 10 minutes. He dribbled through the middle, his quick feet steering the ball through Brentford's defence before he elegantly flicked the ball past Simon Moore with the outside of his right foot. The goal contained hints of Romario in his pomp.
Chelsea's mood was better but Turnbull was hardly a human barricade.
Rosler sent on Tom Adeyemi for Trotta and the substitute had an instant impact, running through and being brought down by Turnbull after 73 minutes. Forrester calmly took control of the penalty, placing it low to Turnbull's right. Benitez's second substitution after 79 minutes involved a right-back, Cesar Azpilicueta, for a right-back, Branislav Ivanovic.
Where was Ba? Sitting on the bench, Where was the boldness of a Jose Mourinho?
Finally, Benitez sent Ba on and he quickly created the stage for Torres to equalise with a superb curling strike. The Spaniard's finesse was a total contrast to his earlier travails.
Chelsea could even have won it in stoppage-time, Mata and Ba screaming for a penalty when Harlee Dean handled Mata's cross but Brentford deserved their replay and a payday that could touch £750,000.
(© Daily Telegraph, London)