1-0 came after just ten minutes when Brentford won the ball high up in United's half.
Josh Dasilva collected the ball and tried a long-range shot that de Gea dropped into the goal.
Eight minutes later came goal - and invitation - number two.
United's goalkeeper was about to start a play and matched an already marked Christian Eriksen.
The Dane lost the ball in the duel with his compatriot Mathias Jensen, who responded by placing the ball in the goal.
Mbuemo put 4-0
3-0 came on corners where new signing Lisandro Martinez lost a header duel with Ben Mee.
The nightmare didn't end there.
Brentford won the ball in their own half and immediately started a long play on striker Ivan Toney.
The Englishman extended to fellow striker Bryan Mbeumo who coolly slotted home the fourth for Brentford.
Ronaldo played from the start
Manchester United lost the opener against Brighton 1–2 at home.
Cristiano Ronaldo was there from the start.
Anthony Elanga is on the bench and Victor Nilsson Lindelöf is not in the match squad for the second game in a row due to a feeling.