Opponents Millwall are currently sixth in England's League One but the London club beat top-flight Bournemouth and Watford in the previous rounds of the FA Cup. They are unbeaten in their past 12 games in all competitions.
"They have beaten two Premier League teams. We know this," Ranieri, whose team needed a replay to get past second-tier Derby County in the previous round, told reporters on Thursday.
"We are the underdogs, if they beat Bournemouth and Watford who are above us in the league.
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"Millwall are a good team -- compact and full of confidence. They play well together but we are ready."
Ranieri will be missing strikers Islam Slimani and Leonardo Ulloa due to injury, but said he planned to keep faith with the team that got Leicester past Derby in the last round.
"I want to give another opportunity to the players who played against Derby," he added.
"We have to play for our fans, 3,000 of them will be there with us to support us and I want to see our players fight.
"My mind doesn't change if I win or if I lose. I stay positive to try to help my players. You have to continue to believe."
(Reporting by Simon Jennings in Bengaluru; Editing by Keith Weir)
