The Reds made a blistering start to their home match with Norwich City on Saturday but were unable to maintain that intensity for the full 90 minutes, losing 2-1.
Grabban scored his sixth of the season to put Aitor Karanka’s Forest into an early lead but he said the goal was insignificant in comparison to the overall result.
“It is nice to score,” said Grabban. “But in the end it is pure disappointment from the game. We want to be winning these games and that would make it nice to score in. But it is a loss at the end of the day and that is all that matters at the moment.
“They didn’t give us a sniff but it is down to us and if we do what we can do, it doesn’t matter what Norwich would have brought to us in the second half.
“We weren’t able to reach the levels that we can reach and that is why Norwich were able to step up. We are going to have to learn, we have no choice in that, and we need to try and do better in the next game.”
And added: “It is a frustrating afternoon. It started off well but we weren’t able to keep it up for the 90 minutes. Eventually they got into the game and took the win so it is disappointing.
“The more important thing is that we still had 75 minutes to go and we didn’t reach those levels of the first 15-20 minutes, which was disappointing.
“But we move on and we try and extend that good period of play in the next game for the full 90.”