Moses Caicedo had opened the scoring for the hosts early in the first half, before goals from Marc Cucurella, Pascal Gross and Leandro Trossard further compiled United's misery.
Bruno Fernandes and Edinson Cavani saw efforts well-saved and Cavani had a goal disallowed, but ultimately it was Brighton's day.
FIRST HALF - CAICEDO OPENS THE SCORING With 15 minutes gone, Graham Potter’s side took the lead. Caicedo’s effort found the bottom corner from the edge of the area after the ball flew through a sea of bodies.
As the action began on the south coast, the opening exchanges were largely played at the hosts' tempo, as United struggled to get hold of the ball.