Sven-Goran Eriksson admitted a season of poor defending had cost Leicester a place in the npower Championship play-offs after their final-day defeat of Ipswich.
City cruised into a three-goal lead by half-time but then conceded two soft goals before they rallied to win 4-2 at home.
Eriksson said: "Of course it is nice to finish the season with a win for the fans, owners and players themselves, but as usual, in parts we were brilliant, passing the ball around and scoring fantastic goals, and then we suddenly switched off.
"Their second goal was a typical Leicester City goal to concede, and we have done that all season. Things like that have cost us at least a place in the play-offs. We have to make that change for next season.
"You can't concede stupid goals if you're going to get promoted. We have conceded too many easy goals.
"We need to build on the good stuff. Our team is good to watch with some brilliant passing and combinations but that is not good enough to get you promoted. If we did not make so many mistakes we would easily be in the play-offs. We have to do it better next season."