Nottingham Forest manager Aitor Karanka said that his players lacked the desire needed to overcome a battling and in-form Millwall side this afternoon.
Two first-half goals, including one inside the opening 30 seconds, left Forest with a mountain to climb and Karanka was left frustrated at his side's performance.
Speaking after the game, he said: "It is easy to explain the game today. The team which wanted to win has won. We have been working for two weeks, knowing that they are dangerous with long balls and with crosses, then in 30 seconds we concede.
"It is difficult to understand. But after the goal we didn't have a reaction and they showed why they are fighting for the play-offs as they were fighting for every single ball.
"We need to learn that if we don't play at the intensity that we have to play then we will lose games, like we did today."
Karanka knows that his side will need to learn from this experience today and take it with them as they seek to climb the Championship table towards the end of the season.
He said: "It is really disappointing. It is something we have to learn for the future. Maybe it is good for us, this game, as we need to understand that we were seven games unbeaten but in the Championship you cannot sleep.
"The following day you can go to Millwall, lose 2-0 against a team with desire and a team that wanted to win every ball and every second ball. By doing these things, they have won the game.
"In the Championship, you can never stop and you have to keep on working to have the desire to win every single game."