Lamouchi felt his side didn’t show enough desire to beat Barnsley, with the home side picking up a vital three points in their quest for survival.
The result means that Forest go to the final day of the season looking for the point that will secure their play-off spot.
Speaking after the game, Lamouchi said: “It was more than disappointing, not only for the goal in extra time, for the performance, the spirit.
“We didn’t play so well and when you run less and don’t win the duels, it is difficult.
“Barnsley’s desire was much bigger than us for the victory. Congratulations to Barnsley and good luck for the next game. For us it was a big disappointment. The scenario is a disaster. But what I have said to the players is that you can’t always get what you want.
“What is good in football, you can say what you want in any language that you want, but if you run less than the opponents then you won’t win. The desire of Barnsley was huge and if they have more quality then they will probably score before.
“We have to be calm and now we have to think about the final game against Stoke. Win or draw and we will play the play-offs then everyone will be happy.”