Brighton outclassed Chelsea on the night, as Ferdi Kadioglu, Jack Hinshelwood and Danny Welbeck struck.
A furious Rosenior said afterwards: "By far (the worst performance since he took charge). It was unacceptable in every aspect of the game, unacceptable in our attitude. I keep coming out and defending of the players. That's indefensible, that performance tonight. The manner of the goals we conceded, the amount of duels that we lost, the lack of intensity in the team. Something needs to change drastically right now.
"We need to look in the mirror. I need to look in the mirror. But I can't keep coming out here and defending some of the things we're seeing. Manchester United, genuinely the result wasn't there, I felt we turned a corner. The general attitude, spirit, determination was lacking a part from maybe three or four of the eleven. That is nowhere near enough for this club. I can't come out and lie. I'll tell the truth. That was an unacceptable performance in every area."
On the goals conceded, Rosenior was particularly angry.
He said, "The goal's terrible, we have an easy header, we miss the header. The passage before the goal upset me even more. I think there were four or five occasions where we can actually show some courage and quality, and take the ball down and actually play.
"We just kicked the ball back to Brighton. You can speak about a lack of confidence or results not going our way, that doesn't represent anything that I ever want to see. And I won't ever see it again.
"The professionalism wasn't there. It's a really difficult night. The most difficult night not even just here, so far, at this magnificent football club, but in my career. Some of the things I witnessed today, I never want to see again."