Leonardo Jardim blamed individual errors after a 4-1 defeat at home to RB Leipzig on Tuesday saw Monaco crash out of the Champions League.
With the Ligue 1 champions now guaranteed to finish bottom of Group G, having failed to win any of their five matches, last season's semi-finalists cannot even qualify for the Europa League.
A pair of Jemerson gaffes left Monaco 2-0 down in the opening 10 minutes and those errors were followed by captain Radamel Falcao clumsily giving away a penalty, leaving a frustrated Jardim to bemoan his side's lapses in concentration.
"We paid a lot… for our mistakes," Jardim said. "We were not at the level required of a competition like this.
"The disappointment is very strong, we had the goal of bouncing back and winning tonight.
"And then our opponent was successful: four goals in three shots on target - but we were not up to the Champions League this season."
Leipzig coach Ralph Hasenhuttl was thrilled to keep his team's qualification hopes alive ahead of a matchday six clash with group winners Besiktas, although the Bundesliga team will, at worst, be in the Europa League.
"I can say without exaggeration that this is our best performance in the Champions League," Hasenhuttl said. "We started as I wanted, making beautiful transitions.
"The only problem is Marcel Sabitzer's injury. He has a very bad [dislocated] shoulder and went straight to the hospital.
"We knew the result in Istanbul [1-1 between Besiktas and Porto]. A Besiktas victory would have been better for us. But we stayed very focused on our game.
"From now on we no longer have our destiny in hand. But we are sure to play in Europe in 2018.
"And I can also promise that we will win our last match. We can be proud of our efforts."
A brace from in-form Germany international Timo Werner helped Leipzig collect their first ever away win in the Champions League.
"Timo was great. He proved that in a one-on-one he is a world-class player," Hasenhuttl added of his star forward.
"It is very rare that he does not score in this situation. He asked me to come off because he felt tired."
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