Leicester are just two points above the relegation places and last won in the Premier League in October
The 65-year-old Italian -- whose side are just two points above the relegation places and last won in the Premier League in October -- added they are entering a crunch period where if they don't rediscover their form they risk becoming the first champions to be relegated the following season.
The Foxes -- who have at least finished top of their Champions League group though a second-string side were whipped 5-0 by Porto this week which Ranieri labelled as shameful -- will endeavour to record only their second win in eight Premier League matches at home to Manchester City on Saturday.
"This part of the season is our key moment," said Ranieri, who even threatened last week to omit two of last season's star turns Jamie Vardy, who is yet to score this term, and Riyad Mahrez.
"The players aren't satisfied, they want to play better and fight more but it's important to fight together as a team.
"Our strength was this, if we made a dream last season it's because we were together, not because we were a big team. With our effort we made a story.
"Do you see the same team from last season? I don't see, we have to react, to come back.
"The recruitment area are working hard but they wait for me to say 'Okay, now I need this, this and this.
"Until then I want to check my players and the only players who will stay with me are the ones who want to fight until the end."
Ranieri, who will learn on Monday who his side will face in the Champions League knockout phase, says even if Manchester City are on a bit of a slide themselves, they have won just four of their last 14 matches, and missing the suspended Sergio Aguero they had strength in depth.
"There are so many champions, there is Kevin De Bruyne, there is David Silva, there is Kelechi Iheanacho, there are so many," said Ranieri.
"It is a big team, fantastic players, but I want to see (them) against us, when we are strong."