Mauricio Pochettino was left ruing Tottenham's sluggish start as they fell to a 2-1 defeat to Leicester at the King Power Stadium.
Two sublime goals from Jamie Vardy and Riyad Mahrez handed Leicester a 2-0 half-time lead as Spurs missed a number of decent opportunities.
Christian Eriksen and Fernando Llorente then went on to squander glorious chances either side of a Harry Kane strike as Spurs slipped to a fourth Premier League loss of the season.
The defeat means Spurs could be 16 points behind leaders Manchester City going into the weekend's action, and Pochettino admits they must be critical of their display.
"The second half we were more aggressive and started to play the game. We were critical of ourselves at half-time and it's difficult to start the way we did and win games," Pochettino told Sky Sports.
"We were so poor in our performance. We need to improve and be critical about our performance, because we did not play the way we are used to.
"We need to assess it and take responsibility. We cannot start like this. We spoke before the game about our focus but we were poor in this phase of the game.
Though Vardy and Mahrez's first-half strikes were spectacular in their essence, Pochettino still felt as though his side could have prevented them.
"The game was even and we created many chances, but we cannot concede the goals that we conceded today," Pochettino added.
"You can do better, and do something to stop them. When you concede, it's not only because of the quality of the opponent, it's sometimes because you need to be aware of some situations.
"It's true we are disappointed and frustrated, but not only for the defeat but also the performance."