Scott McTominay’s header broke the deadlock just before the interval, before Juan Mata and substitute Paul Pogba scored late on to make it 3-0 and give the scoreline a flattering look.
The Seagulls played some great football at times and were the better team for large parts of the last 16 tie, but couldn’t beat Dean Henderson as United followed up Saturday’s Premier League win with victory again at the AMEX Stadium.
Plenty of changes
Ole Gunnar Solskjaer made 10 changes to the side that so dramatically beat Albion on the weekend, with Victor Lindelof the sole survivor from the 3-2 win. Brighton, meanwhile, made nine changes with Ben White and skipper Lewis Dunk keeping their place.
It was United who created the first opening 17 minutes in when stand-in skipper Mata threaded a ball to Odion Ighalo, who took it round Jason Steele but with a tight angle his shot hit the side netting.
But after that Albion got themselves in the game, dominating possession and Henderson had to be alert to deny Alireza Jahanbakhsh from making it three goals in three rounds, saving the Iranian’s low curler towards the bottom corner.
And then, after Alexis Mac Allister had found himself space but could only fire over when off balance, Jahanbakhsh had another good opportunity but couldn’t hit the target from ten yards out.
But having been on top, Albion fell behind a minute before half-time. United, like they did Saturday, profited from a poorly defended set piece when Mata whipped in a great free kick and McTominay had a free run and headed past a helpless Steele at the far post.
Neal Maupay and Leandro Trossard were introduced from the bench early in the second half, and the latter – having hit the woodwork three times at the weekend – had a great chance to equalise after the hour. Jayson Molumby did well to win the ball in midfield and find Pascal Gross, whose cross picked out the Belgian on the penalty spot, but Henderson brilliantly stuck out a hand to deflect the first-time shot wide.
It was to prove a costly miss, as United doubled their advantage five minutes later. An intricate move involving Donny van de Beek and Mata saw the former backheel the ball into the Spaniard’s path and inside the area he guided his effort past Steele for his 50th goal for the Reds.
Substitute Pogba then extended the winning margin with ten minutes to go, firing a free-kick from 20 yards in off the post with the aid of a deflection off Joel Veltman as United got the job done to progress to the quarter-finals.