Defeat at Reims in the Coupe de la Ligue was followed by a setback at fellow Ligue 1 title hopefuls Lille, before a 1-1 draw with Evian took Monaco into the international break.
The cash-rich principality club have started well in their first campaign back in the French top flight, but their recent dip has left them five points adrift of reigning champions and league leaders Paris Saint-Germain, and one behind second-placed Lille.
And Ranieri urged his side to rediscover a winning attitude, starting with Sunday's trip to Nantes.
"I want a reaction from my players after the last three games without a win," Ranieri said. "We have the technical (ability), but without mental aggression, we are in trouble.
"Slowly we (have) lost concentration that was our strong point early in the season. We must react.
"We must find a collective mentality, that's what I expect from this match against Nantes."
Nantes, who sit fourth in the table, have been a surprise package in the league this season after promotion alongside Monaco, with only Lille and PSG coming away from the Stade de la Beaujoire with points this term.
But Ranieri says the Breton club's start to the season has come as no shock to him.
He added: "I'm not surprised by Nantes.
"They have not changed their way of playing. They are always dangerous on counter-attacks (especially) with Filip Djordjevic."
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