If they did not hold the mantle already, Brentford cemented their status as Chelsea’s bogey side with a third straight Premier League win at Stamford Bridge. This past week Mauricio Pochettino acknowledged Chelsea remain far away from where he expects them to be and on this evidence there is still a long way to go. The brutal reality is they have won one of their past 13 home league matches and overcome only Crystal Palace, Luton and Leeds here in the competition this year.

If anything encapsulated Chelsea’s frustrations it was the sight of Jesús Pérez storming into the Brentford dugout to have words with the analyst Bernardo Cueva, a contretemps that triggered the referee, Simon Hooper, taking charge of his first top-flight game since the embarrassing VAR error in Tottenham’s win over Liverpool, to award the Chelsea assistant manager a red card.

Related:Chelsea v Brentford: Premier League – live

Chelsea have spent more than £1bn on furnishing their squad with some of the biggest talents in the game since the Todd Boehly-led takeover last year and until Bryan Mbeumo capped a counterattack after Chelsea committed everyone forward for a stoppage-time corner in search of an equaliser, it seemed the only goalscorer – and ultimate match-winner – would be Ethan Pinnock, who became Brentford’s £3m record buy upon signing from Barnsley in 2019, after a career in non-league.

Yehor Yarmolyuk missed a golden chance to double Brentford’s advantage after they streamed forward four v two with a minute of normal time to spare but Mbeumo added a second soon after. Neal Maupay, 13 months without a goal, raced towards an empty net with the Chelsea goalkeeper Robert Sánchez haring after him and just as it seemed the Brentford striker may have fluffed his lines he located Mbeumo to his left and passed on duties. The goal was cleared by VAR after Mbeumo appeared to foul the Chelsea substitute Ian Maatsen in the buildup.

The first half felt like an exercise of attack v defence but the longer the game remained goalless the more edgy Chelsea became and the more Brentford’s belief grew. Their low block gnawed at Chelsea, whose players often looked up only to discover traffic in the shape of red-and-white shirts before them. Noni Madueke, promoted to the starting lineup, rattled the crossbar after 10 minutes with a fine strike but otherwise Chelsea were constrained to half chances. “We were not nasty or clinical in front of goal,” conceded Pochettino.

Brentford had to suffer but in another sense they enjoyed suffocating Chelsea’s attack, congesting territory close to the 18-yard box. Mbeumo was selfless up front, earning a pat on the back from his manager, Thomas Frank, after buying a throw-in near the halfway line. The sight of Brentford’s left centre-back Nathan Collins pressing Cole Palmer practically all the way to the Chelsea goalline underlined Brentford’s determination to make life uncomfortable.

It was a damning indictment for Chelsea and their latest summer splurge that, aside from the hosts’ early promise, Brentford always appeared the more likely to strike. As was Frank downplaying his side’s performance after recording successive victories for the first time this season. “I think our first half was not that good, average, if not a little less than that,” he said. “In the first half you could see the exciting potential in a Chelsea team. If I was a Chelsea supporter, I’d be positive about them and I’m convinced it will come [for them].”

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Pochettino, without Enzo Fernández and Mykhailo Mudryk owing to minor muscle injuries, made three changes from their entertaining draw with Arsenal. One of those added to his starting lineup, Nicolas Jackson, sent an early shot wide of a post and from there his afternoon did not get much better. On the verge of half-time a Chelsea supporter a few rows behind the dugout made his feelings plain to Jackson. An animated Pochettino responded by offering the fan a sharp-tongued riposte. “In this moment the energy was a little down in the stadium,” Pochettino said. “I said: ‘Stop talking in this way. Come on, we need support and [for the fans] to stay behind the team.’ I was very respectful.”

Brentford had the first real chance of the second half, Sánchez repelling an effort by Vitaly Janelt after the marauding Brentford centre-back Kristoffer Ajer picked out the midfielder with a first-time pass from the byline. Brentford seized the lead when another roaming centre-half, this time Pinnock, feasted on some slack marking by Axel Disasi to bury a header. Mbeumo sent in a hanging cross from the right and Pinnock displayed the most hunger in the box to power in.