Brendan Rodgers admitted Leicester's Champions League challenge had been undermined by players who breached Covid protocols.
Ayoze Perez, James Maddison and Hamza Choudhury were left out of the crucial match against fellow European hopefuls West Ham having reportedly broken the guidelines.
West Ham went on to inflict only Leicester's second away defeat of the season, with a 3-2 victory lifting them back into the top four, just a point behind the Foxes.
Rodgers would not confirm the identity of the players, but the Foxes boss said: "I can tell you there was a breach.
"My objective when coming to Leicester was to create standards on and off the pitch.
"The standards fell way below what we would expect, especially at this stage of the season when we are challenging and fighting. They fell well below that.
"The guys would have been in the squad but as a consequence they weren't. It wasn't ideal but that's how strongly I felt.
"We were coming into a very important game but some things are more important than football. You have to respect what the nation is going through and what our values are.
"From a personal perspective I was bitterly disappointed. But it's a mistake that they've made and I won't hold it against them. It's in the past now."
Rodgers confirmed the players concerned will be back in consideration for next weekend's FA Cup semi-final against Southampton.
Jesse Lingard scored twice in the first half, the opener a superb volley from the edge of the box, and Jarrod Bowen added a third just after the break to put West Ham in control.
But the Hammers have found 3-0 leads hard to hold onto lately, having been pegged back to 3-3 by Arsenal and clung on for a 3-2 win at Wolves on Monday.
This time Kelechi Iheanacho scored twice to set up another tense finish, and Wesley Fofana headed a last-gasp chance wide to the relief of manager David Moyes.
"I'm disappointed with the way we finished the game. We've got to get out of the habit of going 3-0 up and conceding goals," said Moyes.
"That's really important and we've got to do something about it. But overall I'm thrilled with the results we're getting.
"If before the game you'd offered me 3-2 against the team third in the league and going for the Champions League I'd have snapped your hand off.
"There's a real belief that we can go and win games and we've got players who can do that."
Lingard has now scored eight goals in nine appearances since joining on loan from Manchester United, and Moyes added: "He's has been crucial, he's helped the team greatly.
"I didn't expect him to get quite so many goals so long may it continue."