Yann Kermorgant returned to haunt his previous employers and give the Addicks the lead, but Chris Wood levelled matters in the second half.
Charlton restored their advantage at the King Power Stadium thanks to Danny Haynes' spectacular 20-yard winner and this time the visitors held on for the victory.
Pearson was particularly unhappy with his team's defending, although he still felt they did enough to get at least a point out of the game.
"It's a disappointing result," said Pearson.
"I think they came to do a bit of a job on us and they did it well in terms of getting everybody behind the ball and limiting our ability to play the type of football that we do.
"When you concede the types of goals that were conceding it gives us a problem in terms of winning games and that's really the top and bottom of it tonight.
"I don't think we deserved to lose the game but we have. I know the second goal is a good strike but we've not dealt with a situation I'd expect us to snuff out. From a defensive point of view it's certainly avoidable."