The Red Devils crashed out of the Champions League in midweek, losing 2-1 at home to Sevilla.
Mourinho’s side had previously beaten Liverpool and Chelsea at Old Trafford before the shock defeat.
And the United boss launched a stunning 12-minute rant at his press conference on Friday, defending the club’s Champions League exit.
Brighton, on the other hand, are performing well at present, and have only lost one of their last nine games in all competitions.
And Mourinho believes this means Chris Hughton’s side should be feared.
“An isolated defeat doesn’t reflect the period,” he told MUTV.
“The situations are not with big distances [between them].
“We beat Chelsea and Liverpool and then we lost against Sevilla and it looks now that we are a disaster.
“In a club like ours, with our dimension, it is quite easy to make us go from hero to villain.
“With Brighton, that is not happening so their feeling is for sure that they are in a good moment.
“They have a very good team playing with great stability, with happiness according to the results, so it’s going to be hard.”
Just 16 points separate the ten teams in the bottom half of the Premier League.
Brighton currently occupy 11th position.
And Mourinho believes the Seagulls will still be playing top flight football again next season.
“They now have more experience in the Premier League at the highest level and are obviously more confident because of their position in the table,” he added.
“They can see light at the end of the tunnel and are almost there so it’s brilliant work by Chris [Hughton].
“I really believe in them staying in the top division and now they are fighting to go to Wembley in the Cup.”