Cristiano Ronaldo, who made his first start of the campaign, was at the heart of United’s early bid to get in front. He tested David Raya with a dipping shot from 30 yards before things started to go wrong for the Reds.
Josh Dasilva and Mathias Jensen both capitalised on defensive mistakes to put the hosts 2-0 up after 20 minutes. Erik ten Hag watched on as Marcus Rashford looked to stretch play on the left, before Brentford quickly halted any United momentum.
Ben Mee pounced to make it 3-0 on his home debut for the Bees.