The Sky Bet Championship club have confirmed that St Ledger suffered a grade two tear in his knee during training on Friday, causing the Republic of Ireland international defender to miss Sunday's goalless draw with Leeds.
It is a further blow to the 28-year-old centre-back, whose campaign last season was dogged by injury. St Ledger missed the start of last term with a knee injury before a torn hamstring ruled him out for another five months.
Foxes boss Nigel Pearson hopes St Ledger can defy the early diagnosis and return to action earlier than expected. "I'm told he will be out for about six weeks," said Pearson.
"When you pick up an injury like that on a Friday it's not exactly ideal and it's unfortunate.
"You can't do a lot about contact injuries. I'd be annoyed if it was something avoidable, but you can't do much about things like that. It's disappointing for him, but that's football.
"I suppose, historically, he's been a relatively quick healer so I suppose six weeks is being a bit pessimistic, it might be three or four.
"But we've got plenty of options. Paul Konchesky went into the left side of the three (on Sunday), he's played there before so it's no problem."
The loan signing on Friday of Spain Under-21 international Ignasi Miquel from Arsenal provides Pearson with additional defensive cover.