With the defeat, United have now lost three league matches in a season against newly promoted clubs for the first time since the 1989-90 season. "They beat us in the attitude".
Rashford's indecision in failing to pick out Martial or go for goal himself, though, summed up a frustrating night for United with Mourinho quick to show his discontent on the sidelines.
Seagulls boss became the first Brighton boss to win the Manager of the Month award in the Premier League earlier this season.
The German midfielder, Albion's undisputed player of the year, fittingly hit his seventh goal of the season to take the club to the magic 40-point mark.
Their place in next year's UEFA Champions League has already been secured and they still remain on course to finish second, but their latest setback will have fans crimson with rage as the curtain comes down on what has been a disappointing league campaign. Why always Lukaku? You have the answer why always Lukaku. United goalkeeper David De Gea pushed the ball onto the head of Gross and, although Marcos Rojo hooked it clear from underneath the crossbar, goal-line technology confirmed it crossed the line. "Maybe now you understand why some players play more than others and you don't ask why A, B or C don't play more", said Mourinho.
He made a countless number of poor passes and struggled against Brighton's midfield of Propper and Dale Stephens and when Pogba doesn't turn up, you know it's going to be a tough for them to get a result. "That's what happened - it's normal and we couldn't score a goal and get a point".
"I didn't succeed in persuading my players that the four points we need to finish second are very important", he said.
"When individuals don't perform at a good level it's hard for the team to perform at a good level".