England star Rashford was handed a rare chance to play down the middle with Lukaku injured for the Premier League clash with the Seagulls.
But the 20-year-old suffered a shocking display as United crashed to a 1-0 defeat, and afterwards Mourinho failed to hold back.
He said: "I thought for some of them the possibility to start a game, the possibility to give reason to the ones that during the season there is criticism to my choices -'Why always Lukaku?'.
"You have the answer why always Lukaku."
Mourinho has been bristling throughout the season after facing repeated questions why Lukaku was always leading the line down the middle.
Many wanted Rashford to play in the position he enjoys best, while others have called for Anthony Martial to also be given a better chance.
But both flopped when their moment came against Brighton with insipid displays, and Mourinho made no attempt to cover up his frustrations.
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He added: "Now you see why some players don't play all the time.
"Maybe now you will not ask questions why some are not playing every week."
Mourinho rested several key players as he prepares for the FA Cup final against Chelsea later this month.
But his fringe players blew their chance to stake a claim for a place in his plans at Wembley, and the United boss questioned whether they had the right attitude for the game.
He added: "The attitude from first minute and especially during the first half, they were obviously stronger, they had more appetite, felt the game with different emotions.
GLARING ERROR Rashford fails to square ball to Martial and Mourinho's face says it all
"I didn’t succeed in persuading my players that the four points we need to finish second were important. That didn’t matter to the players.
"To me it does, but for some of my players it doesn't.
"The points Brighton needed meant enough for them to give everything they have to give. They got the bonus of their attitude."
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