Blues fans turned on head coach Rosenior on Tuesday night (Wednesday AEST) at the Amex Stadium, where the club's slim hopes of Champions League qualification were dented by a fifth successive Premier League loss without scoring.
Rampant Albion leapfrogged Chelsea into sixth place thanks to goals from Ferdi Kadioglu, Jack Hinshelwood and substitute Danny Welbeck.
Rosenior was the subject of X-rated chants from the disgruntled away end during the second half of a toothless display in which his team failed to register an attempt on target.
Asked whether the performance was the worst of his tenure, the former Strasbourg boss told Sky Sports: "By far. It was unacceptable in every aspect of the game, unacceptable in our attitude.
"I keep coming out and defending 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.
"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."
Chelsea are in danger of missing out on European football altogether after suffering five consecutive league defeats without scoring for the first time since 1912.
The Blues remain seven points adrift of fifth-placed Liverpool, having now played a game more.
"We need to look in the mirror; I need to look in the mirror," continued Rosenior.
"The general attitude, spirit, determination was lacking apart from maybe three or four of the XI.
"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."
In contrast to the visitors' slump, in-form Albion have taken 19 points from the last 24 available in their quest for continental qualification.
Home fans gleefully rubbed salt into the wounds of the visitors by chanting in support of ex-Seagull Rosenior and boss Fabian Hurzeler savoured "one of the most complete performances" of the season.
"I'm delighted with the performance and I'm very proud of the players," he said.
"I think it was one of the most complete performances in the season and that puts the benchmark quite high."
Chelsea now turn their attention to an FA Cup semi-final against Leeds United on Sunday at Wembley Stadium.
With AP.
