Ben Mee then exposed the visitors’ frailty from set-pieces for his first Brentford goal and Bryan Mbuemo rounded off a stunning counter-attack 10 minutes before half-time to sink United to the bottom of the table for the first time in 30 years.
The jubilant home support taunted Ten Hag with chants of “you’re getting sacked in the morning.”
But the Dutch coach has inherited a rabble rather than being the root cause as United lost a seventh consecutive away league match for the first time since 1936.
That run included shipping four goals at Manchester City, Liverpool and Brighton last season.
The arrival of £57 million ($69 million) centre-back Lisandro Martinez from Ajax has done little to shore up United’s leaky defence and questions will be asked of why Ten Hag has left Raphael Varane on the bench in favour of the Argentine and the out-of-sorts Harry Maguire.
Ten Hag did recall Cristiano Ronaldo to the starting line-up, but the wantaway five-time Ballon d’Or winner played a part in the opening goal.
Ronaldo went to ground too easily seeking a foul as he was surrounded by Brentford bodies. Dasilva’s shot should still have been a comfortable save for De Gea, but the ball squirmed underneath the Spaniard.
De Gea compounded that mistake eight minutes later with an attempted pass to Christian Eriksen, who was outmuscled by his former teammate Jensen, and the Dane coolly slotted home to double Brentford’s lead.
Martinez’s diminutive 5ft 9in (1.8m) frame was raised as a red flag for his ability to adapt to the physicality of the Premier League.
And the Argentine was beaten at the back post by Mee for the third goal after Ivan Toney headed a corner across goal.
Credit to Brentford in just their second season in the English top-flight for 75 years may get swept away in the waves of criticism coming United’s way.
But Thomas Frank’s men produced a stunning move to tear United open again for their fourth goal.
Toney was released down the left wing and picked out Mbuemo with a precise first-time pass for the Frenchman to race in behind Luke Shaw and finish low past De Gea.
Ten Hag made his feelings clear at the break with a triple substitution with Shaw, Martinez and Fred the scapegoats.
United did at least stop the bleeding after the break and should have had at least one consolation as Ronaldo looped a header over before Eriksen’s header lacked the power to beat David Raya.
The damage, though, was long since done for Ten Hag and worse could still be to come.
United host Liverpool, who beat their historic rivals 5-0 and 4-0 last season, in their next game on August 22.
(This story has not been edited by NDTV staff and is auto-generated from a syndicated feed.)
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