The Red Devils were beaten 3-2 by Chris Hughton's side in their second game of the Premier League season, with goals from Glenn Murray, Shane Duffy and Pascal Gross doing the damage for the hosts.
And Mourinho was clearly frustrated with what he saw from his side over the 90 minutes, appearing to take offence when a reporter asked about an apparent lack of chemistry among his players.
The beIN SPORTS reporter first asks: "Manchester United are a team with a big reptutation. Was that performance befitting of a Manchester United side?"
With Mourinho offering a short answer in response: "We made mistakes. And we were punished by the mistakes."
The journalist then follows up by asking: "It didn't seem that there's enough communication and enough chemistry among your team. Is that fair to say?
"Even with the substituons in the second half, the quality going forward, the communication..."
The Portuguese, seemingly unimpressed with the choice of questioning, says in reply: "You must be fantastic in your job to have the ability from the stands to speak about chemistry between players.
"We made mistakes we were punished by the mistakes, I cannot say more than that."
Juan Mata also offered his reaction to the defeat, following his manager's lead in blaming mistakes for the loss to the Seagulls and insisting the journey back to Manchester was a difficult one.
He wrote in his weekly blog: "Without doubt, it wasn’t a good Sunday and the motivation to write something isn’t really there. The journey back from Brighton was tough, as you can all imagine.Both for us and for everyone that was there to support us.
"Although things have only just begun and there is a long way to go, it always hurts to lose. A lot. We made mistakes and they penalised us for them.
"We have to pick ourselves back up, improve and be positive, because there’s plenty still to come and feeling sorry for ourselves isn’t going to help anyone. Thanks for all of your support, as always."