Forest were guilty of spurning good chances in a goalless first half which offered some encouragement to the travelling fans following Saturday’s dismal defeat at a struggling Rotherham United.
Then three quick-fire goals in the second half at Hillsborough saw the Reds crash to a 3-0 loss to leave O’Neill’s team six points off the top six with just five games of the season yet to play.
And the manager was under no illusions at the issues he needs to address at full time.
“We are really disappointed. In the first half we were excellent and could have scored a couple of goals, but it always comes back to haunt you,” said O’Neill.
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“There was a mad spell at the start of the second half when we conceded a goal from our own throw, the second from our own set piece and it was disappointing.
“Unfortunately, we showed a brittle side of us again and we lost, maybe it was a bit harsh but we have been beaten by three goals when it could have been so different.
“We have broken on them on a number of occasions and I thought we played some delightful football. We had played coming in, Arvin [Appiah] doing his best and Sam Byram coming in and doing fine.
“We have got these chances that we have spurned and we can’t keep doing that. I don’t care what league you are in, you can’t spurn these chances. Of course, when we don’t get the first goal then the confidence saps.”