Beleaguered manager Claudio Ranieri has called for Leicester's "gladiators and soldiers" to step forward and save the club's season following yet another defeat.
Millwall progressed to the sixth round of the Emirates FA Cup at Leicester's expense after Shaun Cummings scored in the 90th minute at the Den to pile yet more misery upon Ranieri and the defending Premier League champions.
Leicester had an extra man on their League One opponents for the majority of the second half after Jake Cooper was sent off, having been shown a second yellow for a rash challenge on Ahmed Musa.
But the deficit only sought to inspire Millwall to the win with Cummings latching on to a knock-down from substitute Lee Gregory to coolly slot past Leicester goalkeeper Ron-Robert Zieler.
Leicester, one point above the Premier League relegation zone and now out of the FA Cup, are without a victory from their last eight matches.
"I want to speak again with the players and say we have to fight every match," said an animated Rainieri. "'Who wants to fight? Tell me'. I need the soldiers, I need the gladiators, because Millwall, with 10 players, showed they are fantastic gladiators.
"It is strange because last season we won for this, to be more determined than the opponent and play with more heart than the opponent. We could also lose but we would fight every match. I want to see this, the fight until the end.
"This defeat could be good for us because when you lose against a team who fights, you have to say well done to them but why did we lose?"
With one eye on their trip to Sevilla in the Champions League next week and the visit of Liverpool the following Monday, Ranieri made ten changes to the side which lost at Swansea.
Leicester started well, but failed to find a way past Jordan Archer in the Millwall goal, and then failed to take advantage of Cooper's 52nd-minute dismissal.
"Going down to 10 men just after half time is tough," Millwall manager Neil Harris said. "It put the pressure and expectation on to Leicester's shoulders and my players did not freeze. It galvanised the stadium.
"The atmosphere in the stadium was electric. It carried the players and my players carried the stadium because of the pressing with 10 men, the closing down and the chances we created. It was everything you would want from a Millwall team."
As Millwall celebrated a famous win there were ugly scenes at the final whistle after the home supporters spilled on to the pitch and goaded the Leicester fans. Missiles appeared to be thrown by both sets of supporters while police horses were called upon to defuse the situation.
"No-one wants to see that on the football pitch," Harris said. "I can't condone that, but if I have not seen it I am not going to say anything negative because you have to see something to make a realistic judgement.
"What I will say is my players were excellent today and our fans were brilliant during the 90 minutes."