MANCHESTER, England -- Manchester United manager Erik ten Hag has attempted to brush off the boos that greeted his decision to substitute Rasmus Højlund during the 3-1 defeat to Brighton.

A disastrous day for Ten Hag was made worse when his decision to replace Højlund with Anthony Martial in the second half prompted loud boos from the home support. Afterwards, he insisted the reaction was down to Højlund's positive performance on his full debut.

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"I think it was positive," Ten Hag said. "You see the fans in the first moment in Old Trafford, the reception for [Højlund] was great. I think he performed very well. It's good they give this signal, this message. It will give Rasmus belief."

Højlund, 20, almost marked his first United start with a goal but had a first-half equaliser ruled out by VAR. The 72 million ($89m) summer signing had to wait to make his debut after arriving from Atalanta with a back injury and Ten Hag said he was not capable of playing the full 90 minutes against Brighton.

"Everyone knows he came in with a small issue," he added.

"We built him over the last three or four weeks but he is not ready for a whole game and we have many games to play at short notice, so we have to build the fitness.

"If he gets injured because he is not capable of playing 90 minutes then we are much farther away than where we want to be."

Ten Hag was also quick to dismiss a suggestion the boos were for Martial, who struggled to make an impact after coming on.

"I think it was because Rasmus Højlund came off but I don't want to discuss that so I don't know," said Ten Hag.

"I think Anthony Martial last year every time he played he had a very positive effect on our games. We even almost lost no games with Anthony Martial on the pitch, so if that's the fact, then I don't understand because when he is fit he will definitely contribute."