Brendan Rodgers slammed Leicester City after a 4-1 loss to Bournemouth, labelling his side's performance as "not acceptable".
Chasing a top-four finish in the Premier League, Leicester led through Jamie Vardy before falling apart on Sunday.
Junior Stanislas scored a penalty, Dominic Solanke a brace and Jonny Evans an own goal as Caglar Soyuncu was sent off, and Leicester will fall to fifth in the table if Manchester United beat Southampton on Monday.
Rodgers was furious with his side, who have won just one of seven games in all competitions since the season resumed last month.
"It's not a happy changing room and it shouldn't be, because that is not acceptable, to deliver that level, especially when you work so well," the Leicester City manager told LCFC TV.
"Like I say, it was the attitude. We didn't show the right spirit and that's something that we've showed consistently. But, yes, that was bitterly disappointing.
"[The Sheffield United game on Thursday] will be a great challenge for us because we need to show a greater level of physicality and mentality. That'll be a really good test.
"We had a great result at Bramall Lane earlier in the season and now this is a game where we've really got to show our mettle and look to get working again."
After hosting Sheffield United, Leicester have games against Tottenham and Manchester United to finish the season.
Rodgers apologised to the club's fans for his side's performance, lamenting the second-half showing.
"Listen, first of all, I apologise to the supporters. I don't think I've ever had to do that in my time here," he said.
"We've always been a tough team, mentally and physically, and tried to then play football, but to collapse like we did in the second half was bitterly disappointing."