Radamel Falcao, who returned from injury as a substitute, missed a penalty as the Ligue 2 champions suffered a shock 2-1 reverse at home to the relegation candidates on Friday
An Eric Abidal own goal and a strike from David Ducourtioux sealed the three points for the visitors at Stade Louis II, despite a late header from James Rodriguez.
Moreover, Radamel Falcao missed a second-half penalty on his return from injury as Monaco missed the chance to move temporarily top of Ligue 1 and Ranieri was left to rue his side's profligacy in the defeat.
"I am not angry. Matches are always difficult," he told a press conference after the defeat.
"I think we played poorly in the first 45 minutes. It was better after the break, we created chances.
"Perhaps we lacked maturity. Had we scored just a quarter of our chances, it would have been 4-2 - but this is football."
League leaders Paris Saint-Germain can now move five points clear heading into the winter break should they beat third-placed Lille on Sunday.