The principality club lost their unbeaten Ligue 1 run following a 2-0 defeat at the Stade Pierre-Mauroy which sees them fall below Lille to third in the table.
"I'm very proud of my players," Ranieri told reporters. "They played a great match. We had a lot of chances, but we just couldn't score."
Monaco return to Stade Louis II on Friday where they host Evian TG.