Bruno Fernandes and Cristiano Ronaldo, the latter with a penalty, also netted on Monday, which is the last home fixture of a difficult campaign for United.
United remain in sixth place with 58 points from 36 games, five behind fourth-placed Arsenal but having played two more matches. Brentford have 40 points from 35 games and may need one more win to ensure they not dragged into a relegation fight. But there was no mass exit as the vast majority stayed to watch what was a rare commanding performance by United in recent months.
“We know our standard has not been at everyone expects and our own expectations were bigger than this.