Nottingham Forest manager Steve McClaren says his side let the club's supporters down in losing 2-1 to 10-man Derby County at the weekend.
The Reds led 1-0 after only two minutes thanks to an Andy Reid penalty following the dismissal of Derby goalkeeper Frank Fielding.
McClaren told BBC Radio Nottingham: "We let the fans down.
"Never mind ourselves as individuals and a team, the fantastic support here deserve better - it is unacceptable."
Forest are now inside the Championship relegation zone with only one win in their opening seven matches.
McClaren added: "We were totally lacking in the aspect of mentality and character.
"We know we have difficulties and we will have to address them, there's no hiding place.
"Whatever stick we get we deserve it and we have to regroup and look to Tuesday and make it never happen again."
Forest play Premier League Newcastle on Tuesday in the third round of the Carling Cup, followed by a trip to Watford in the Championship on Saturday.
"Hard work on the training ground can get it right for us," McClaren said.
"Football is a cruel game and I have been hurt and had down times many times before, I hope this is the lowest we can go.
"I said before it is work in progress, I should've said it is a lot of work in progress."