Claudio Ranieri urged Leicester City to heed the example set by Millwall's "gladiators" in the League One side's stunning FA Cup triumph over the Premier League champions.
The Foxes were dumped out of the competition by the 10-man Lions, with Shaun Cummings' last-gasp goal the difference at The Den.
It provided a new low point in Leicester's poor campaign, which sees them fighting for top-flight survival.
Ranieri fears his side have lost the fighting spirit that underpinned last season's remarkable title triumph and called on his players to learn lessons from Millwall's resilience.
"It's strange because last season we won for this, being more determined than the opponent, playing with more heart than the opponent," he said. "We could also lose, but we'd fight every match.
"I want to see this. To fight until the end. I think this defeat could be good for us because when you lose against a team who is there and fights, you have to say well done to them but why have we lost? We lost for this, this and this - and then we can take what Millwall show and take for us.
"I want to speak again with [his players] and say 'we need to fight every match. Who wants to fight, tell me?' Because I need the soldiers, I need the gladiators. Because Millwall [when they were reduced to 10 men], showed fantastic gladiators.
"They showed more character, more desire, more heart than us. At the end they deserved to win."
Many home fans invaded the pitch at the end of the match, with some heading towards the stand housing Leicester's supporters.
Reports that objects were thrown have not been confirmed, though police on horseback were deployed to prevent the situation developing into something more serious.
However, Lions boss Neil Harris insists he saw nothing untoward.
"It's a big moment for us so I would expect to see people on the pitch," he said. "I didn't see anything being thrown so I won't comment, but obviously I don't condone anything like that as Millwall manager."