Paul Pogba and Alexis Sanchez started Saturday's showdown on the bench, as Jose Mourinho instead preferred Scott McTominay in the middle of the park and Anthony Martial in attack, per ESPN FC's Rob Dawson:
Meanwhile, Sergio Romero retained his place as Mourinho's FA Cup goalkeeper, and Luke Shaw returned to face a team the Red Devils had defeated 1-0 at Old Trafford in November.
The Seagulls enjoyed a good share of possession early on, but it wasn't long before United fell into their usual pattern of dominating possession—at least against "smaller" teams.
Brighton's front three of Leonardo Ulloa, Jurgen Locadia and Solly March made attempts to rustle the United back line, and while the home side had more possession, they struggled to turn that pressure into chances.
Goal's Sacha Pisani saw Chris Hughton's visitors as the better side inside half an hour, as Brighton sat deep to protect their own lines above all else:
Both teams' central defenders had big chances denied around 30 minutes in, when Lewis Dunk tested Romero at one end and Chris Smalling headed off the outside of the post from a corner down the other.
Lukaku finally ended the inaccurate string of attempts, meeting Matic's first-time cross at the far post to head in. James Ducker of the Telegraph noted that for all the criticisms, the Belgian has enjoyed a fruitful first term at Old Trafford:
Ashley Young replaced Shaw at left-back at half-time after the latter surrendered territory down his flank several times in the first period.
After leading the shot count 7-5 at half-time, United failed to register an attempt at the Brighton goal in the first 25 minutes of the second half, a period in which the Seagulls managed eight, per WhoScored.com.
The travelling fans could afford to feel some promise around that improvement, especially when their team came close to scoring through Pascal Gross, while nerves grew that the United contingent might struggle to get a second.
Tim Krul was hardly tested in the second half but was beaten when finally called upon in the 83rd minute, failing to prevent Matic's header at the back post from meeting its target. Pisani felt the scoresheet aptly highlighted the best players on the day:
The introduction of Young as free-kick taker had its impact and directed some credit for the victory toward Mourinho's tactics.
Brighton's hope of an FA Cup fairytale ended at the Theatre of Dreams, with the two remaining semi-final berths to be decided when Wigan Athletic host Southampton and Chelsea travel to Leicester City on Sunday.