United flew out of the blocks in a bright start but fell behind in the 20th minute to Brighton. Danny Welbeck played the ball wide to Simon Adingra and ran into the box, with Adingra’s low cross stepped over by Adam Lallana before the United academy graduate struck.
Marcus Rashford saw a shot deflected on to the woodwork as United sought a leveller that they thought had found in the 40th minute. Rashford cut the ball back for full debutant Hojlund to steer home what would have been his first for the club, only for the VAR to rule the ball had gone out of play.
Former Premier League referee Mike Dean, speaking on Sky Sports News, said: “It’s very tight I can understand why the assistant didn't see it. It took a lot of freeze frames and forward and backwards but the ball had just gone out so a correct decision by VAR Chris Kavanagh.”
Hojlund was handed his first United start and Sergio Reguilon made his debut as Brighton visited Old Trafford.
The 20-year-old striker looked promising off the bench at Arsenal and led the line on Saturday, when Reguilon came in at left-back.
Scott McTominay was the other change in place of Anthony Martial, Aaron Wan-Bissaka and Antony, who was allowed a leave of absence following assault allegations made against him.
Brighton made six alterations as Jason Steele, Tariq Lamptey, Mo Dahoud, Adam Lallana, Danny Welbeck and Simon Adingra came in. Ansu Fati was among the substitutes.