Hojlund has looked more threatening in his two appearances for United than Martial has looked in years. The boos were a release of frustration, knowing that the Brighton defenders would've been breathing a huge sigh of relief to see Martial replacing Hojlund.
Just before the sub was made, a fan sat behind me was saying "he'll take Mctominay off and play Martial up front with Hojlund". Had he made a more attacking sub like that then there wouldn't have been any boos. Considering we were chasing 2 goals and Hojlund was such a handful, I still can't understand what Ten Hag was thinking by subbing him off for someone as anonymous as Martial.