Aitor Karanka said it was a poor game and a poor performance from his side as they fell to a 3-0 defeat against Preston North End.
The Forest manager was left embarrassed by his side's display and gave an honest assessment of his team after the game.
He said: "I am embarrassed as I can understand the fans who left early. I had to thank the fans who stayed until the end as it was a poor game and a performance.
"If we play like that it is impossible to win games in any league, especially in the Championship and especially when you make similar mistakes.
"The first half and second half we looked like a team without soul and without spirit. It is unbelievable as to think 10 days ago we played against Wolves in that way.
"I am not used to having games like this. It is not about attitude or characters on the pitch; you can lose and you can win but you have to at least leave the pitch thinking you have done everything.
"I don't think all the players can go home thinking they have done their best."
A lot of hard work needs to be done on the training pitch and Karanka is determined to put things right.
He added: "I need to think a lot, analyse everything and to keep going forward as this club doesn't deserve performance like that.
"I am the only one who can transmit my passion, my love for the club and for the game so we have to work to put this club where it belongs.
"We have to improve, keep going and work hard. We have tomorrow to bring some more players in and we will continue to work."