Robbin Ruiter admits that Sunderland did not have the same fighting spirit at Bramall Lane that they showed to earn a point at Wolves.
The Black Cats were heavily beaten by Sheffield United in what was arguably their worst display of the season.
The Dutchman has called upon his side to revert to the improvements shown under the new manager so far and take three points at the City Ground on Saturday.
Ruiter said: “We were definitely not ourselves after the recent improvements. It was nothing like we did over the last couple of weeks. There was no fighting spirit like we had against Wolves. All that spirit was only around for 10-15 minutes in the second half.
“We have got to talk about it and move on as quickly as possible because it is a big week. Saturday we have Forest away and then Monday we have Barnsley at home, so we have got to move on quickly from this. This might never happen again, this was way below our levels. We have to build on what we have done in recent weeks and not at Sheffield United. “We have to look at ourselves and not our opponents. We have been busy, we have trained hard and we just didn’t do it at Sheffield United.
“We have to show character, but let’s not start to be negative all over again. We had a really tough start to the season, but when the new manager came in I think we have started to play better. This is the first time since he came in that we have played as badly as we did. We have to step up and be bright, show everyone that this was just one incident and we won’t be like this every week.
Ruiter admits he was at a loss to explain the performance but rejected any suggestion that fatigue or inexperience had been a factor.
He said: “If I would have known what went wrong I would have told everyone on the pitch. I just don’t think we had the fighting spirit against Wolves. That was missing. We need to fight for every single ball or we are not good enough.
“Tell me why it wasn’t there? If I could say so then it would be easy to resolve. We have to be really critical of each other because we can learn from this. This might never happen again. We need to show what we did in the last couple of weeks but not at Sheffield United.
“Everyone in the squad is good enough to play for Sunderland. The other day Maja scored a brilliant winner, so we will not have any excuses about age or fatigue.
“Everything has been really positive [before],” he added. “I don’t think that we are only going to make steps forward.
“We fell enough this year but every time we have slipped we have had to stand up. We certainly have to stand up on Saturday and take the points back.”