Erik ten Hag's side had lost six times across all competitions, including two consecutive defeats at home, before they welcomed the Bees.
United made a terrible start by going 1-0 down to the struggling west London side on Saturday and questions were asked of goalkeeper Andre Onana once again.
The 27-year-old emerged as one of Europe's top goalkeepers during his time at Inter Milan but has been poor so far at Old Trafford.
United have struggled in defence since the start of the season and Brentford were on hand to exploit their fragility.
Victor Lindelof failed to clear his lines and the ball fell to Mathias Jensen, whose shot slipped under Onana.
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Speaking on commentary for talkSPORT, ex-Arsenal winger Perry Groves said: "It's the stuff of nightmares. Casemiro gives the ball away, it comes out to Wissa and then there's six Man United defenders around the ball, none of them close it down.
"I would say schoolboy defending but that would be an injustice to schoolboys."
It is the 18th goal Onana has conceded during his United career thus far and fans on social media have questioned his skills again.
"Onana just genuinely looks awful. Like banter and the rest of it aside… What an earth has has happened to the keeper they signed from Inter?" asked one on X.
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Another added: "Casemiro killed us again but Onana should be saving that! Shocking."
talkSPORT's Mark Goldbridge said: "Onana must have the weakest wrists in the Prem..."
As the half-time whistle blew, a disgruntled Old Trafford expressed their disgust with boos.
However, late substitute Scott McTominay scored two goals in second-half stoppage-time to win all three points for United in dramatic circumstances.