The Chelsea boss came under fire from his team’s fans in their 3-0 defeat by Albion at the Amex.
Home supporters responded by chanting Rosenior’s name and singing: “He’s one of our own.”
Rosenior played for Albion during their ultimately successful bid to win promotion from the Championship to the Premier League a decade ago
He also did some coaching work with the under-23s.
But he had a tough night back at the Amex as his side were outplayed.
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.
They could even be heading for a finish in the bottom half of the table.
Rosenior, who also spoke bluntly in his press conference for the written media, added. “We need to look in the mirror. I need to look in the mirror.”
Both teams were missing key players and Chelsea’s absentees included attacking trio Cole Palmer, Estevao and Joao Pedro.
Former Albion trio Marc Cucurella, Robert Sanchez and Moises Caicedo were all booed by home fans with the loudest such treatment reserved for the former.
Sanchez was mocked for his wayward kicking although he made four fine saves.