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Featured Image Credit: Sky Sports
The Bees started the much better side and within 10 minutes, found themselves in front thanks to Josh Dasilva's second goal in as many games.
Cristiano Ronaldo lost possession and the ball was worked to Dasilva, who decided to have an effort from long-range.
It was a speculative effort at best, but de Gea somehow let it squirm under his body and into the back of the net.
The Spain international has been criticised for his lack of ability on the ball but is still regarded as one of the Premier League's best shot stoppers.
He hasn't shown it this evening however and fans were quick to react on social media.
One person said: "David de Gea earns £350,000-a-week. The highest paid goalkeeper in the world."
Another put: "Has he got holes in his gloves."
A third tweeted: "United needed to make a call on this guy a long time ago."
While someone added: "He could be the worst goalkeeper in the league now, a sad decline."
Thomas Frank's side doubled their lead eight minutes later after a mix-up between Christian Eriksen and de Gea let Mathias Jensen turn home.
BRENTFORD HAVE A SECOND! ⚡⚡— Sky Sports Premier League (@SkySportsPL) August 13, 2022
It's gone from bad to worse for Man Utd, another error at the back, another goal conceded. Jensen gets this one... 🐝 pic.twitter.com/ENgmAsnO0G
Eriksen joined United as a free agent in the summer after spending the second half of last season with Brentford.
Some home fans jeered when he touched the ball and chanted 'Christian, what's the score?' at various points in the half.
Much has been made about Brentford's physicality from set pieces and it came into play for their third goal.
Ben Mee, a summer arrival from Burnley, scored his first for the club on the half hour mark after an almighty struggle in the penalty area.
BEN MEE MAKES IT THREE! 🚨 🤯— Sky Sports Premier League (@SkySportsPL) August 13, 2022
Brentford "pile the misery" on Man Utd. 3-0, half an hour gone. pic.twitter.com/oSjoZgU3ux
United made one change from Sunday's 2-1 defeat to Brighton, bringing in Ronaldo for Scott McTominay.
He failed to have much of an impact however and while attempting to reduce the deficit, United were caught again.
Jensen managed to dispossess Jadon Sancho in his own box and played the ball out to Ivan Toney.
⭕ "BRENTFORD ARE DEMOLISHING MAN UTD!" ⭕— Sky Sports Premier League (@SkySportsPL) August 13, 2022
Bryan Mbeumo adds a fourth, this is simply unbelievable! 🤯🤯🤯 pic.twitter.com/wcIKWHqnY9
Toney slipped the ball through to Bryan Mbeumo, who finished a slick counter attack after holding off Luke Shaw to make it four inside 35 minutes.
If the result stays as it is, United will end the day bottom of the Premier League.