Ruben Amorim's side paid the price for another error-strewn display as Igor Thiago netted twice in the opening 20 minutes to punish shambolic defending.

Benjamin Sesko reduced the deficit with his first goal for United before half-time.

But United captain Fernandes squandered a golden opportunity to rescue a draw when his tame penalty was saved by Caoimhin Kelleher late in the second half before Mathias Jensen wrapped up the points in stoppage-time.

Amorim had arrived in west London hoping to finally win successive league matches as United boss.

Instead, he left fearing the sack after the latest low moment since he moved from Sporting Lisbon to replace the axed Erik ten Hag in November.

United have lost three of their six Premier League games this season, winning just twice, and are languishing in 13th place.

A humiliating League Cup loss at fourth-tier Grimsby in August had fuelled speculation over Amorim's future, with the boss admitting he had thought about quitting and often hated his players.

The 40-year-old was under fire again after miserable 3-0 loss at Manchester City before last weekend's 2-1 win over Chelsea gave him a little breathing space.

Losing to a Brentford team beaten three times in their first five games under inexperienced boss Keith Andrews was a fresh indignity for Amorim, who will face renewed questions about his ability to revive United.

United co-owner Jim Ratcliffe reportedly gave his backing to Amorim during a recent meeting at the club's training ground.

Ratcliffe's support is likely to waver after this result, which leaves Amorim in dire need for a win over promoted Sunderland at Old Trafford next weekend before a daunting trip to champions Liverpool.

Matheus Cunha returned to the United team as the Brazilian started along with Benjamin Sesko and former Brentford forward Bryan Mbeumo in attack for the first time since the trio's summer moves.

But United's recent trips to Brentford have proved painful experiences, with a 4-0 defeat three years ago and a 4-3 loss in May.

- United in disarray -

Their latest visit to west London was no exception as United's flaws were quickly exposed when Thiago struck after just eight minutes.

Jordan Henderson played a fine lofted pass beyond United's flat-footed defence and, with Harry Maguire's offside appeal in vain, Thiago drilled a clinical finish past Altay Bayindir from just inside the penalty area.

Brentford struck again in the 20th minute when Thiago flicked on to Kevin Schade, who burst into the area for a shot that Bayindir could only palm to Thiago, who poked home from close-range.

Signed from Club Brugge to replace Bryan Mbeumo, Thiago already has five goals for the Bees, while the United forward, who netted 20 times in the league last term, has just two this season.

Like Mbeumo, Sesko had endured a disappointing start following his move from Leipzig.

The 22-year-old Slovenian finally showed a glimpse of his potential in the 26th minute.

Kelleher failed to deal with Patrick Dorgu's cross under pressure from Mbeumo and Sesko converted the rebound after his first two efforts were saved.

Yet United's sloppy defending almost cost them again as Diogo Dalot's weak challenge in the area was shrugged off by Kevin Schade, who teed up Dango Ouattara for a low shot that Bayindir saved at full stretch.

Amorim was given a lifeline in the 75th minute when Nathan Collins' tug on Mbeumo conceded a penalty.

But after a long delay for a VAR check and a host of substitutions, Fernandes took a weak spot-kick that Kelleher easily saved.

As the final seconds ticked away, United were caught with sucker punch on the counter.

Jensen raced away unchallenged and cut inside for a ferocious strike that fizzed into the roof of the net as Amorim's misery mounted.