The Seagulls were 3-0 down at one stage due to goals from Matheus Cunha and Bryan Mbeumo either side of a Casemiro effort which was heading wide until it went in via Yasin Ayari.
Danny Welbeck’s free-kick and a soaring header by Charalampos Kostoulas revived the visitors before Mbeumo added his second with almost the last kick of the game.
Albion boss Hurzeler said: “It is easy to summarise this game. Too many easy mistakes on our side.
“We gave the balls too easily away and, like this, you can't win the game.
“I think overall we have the control of the game, we have good actions.
“We were always a danger, we create always a danger in their half and their box but, in the end, if you do too easy mistakes like we did, and give them like four presents, you can't win the game.
“Of course, United pressed well in the first stuff but then you have to do the small margins right.
“It's about which foot you pass with, how are your positioned when you receive the ball, how you find a free team-mate, how calm you stay, the execution.
“If you face a team that presses man to man, it is all about execution, it's all about the angles you create.
“We weren't on our highest level and then we made mistakes and then, how the goals went in, on the one side was luck and on the other side was individual quality.”