The Sabres provided the update hours after Eichel was hurt in the team's final practice before it opens the regular season hosting Montreal on Thursday.
"As of now, no timetable has been set for his return," the team posted on its Twitter account.
Eichel was in front of the net when he got tangled up with a defenseman and his leg buckled beneath him.
He lay on the ice and grabbed his leg in pain as the team circled around him. Eichel was unable to put any weight on his leg as teammates helped him off the ice.
The injury occurred with less than 10 minutes remaining in practice, coach Dan Bylsma said.
"It's the cusp and eve of starting a new season, one we are all looking forward too, and I think that's why it takes the wind out of the sails of the arena today," Bylsma said.
Eichel is key to the Sabres future after being selected with the No. 2 pick in the 2015 draft. He is coming off a solid rookie season in which he had 24 goals and 32 assists for 56 points in 81 games last year.
Eichel played one year at Boston University and became the second freshman to win college hockey's top honor, the Hobey Baker Award.
"You expect to have to deal with injuries throughout the year," Bylsma said. "It's one we are going to have to deal with, whatever the extent of the situation is. Our team is not just Jack Eichel, it's not just Ryan O'Reilly."
O'Reilly, the Sabres' captain, participated in his first full practice Wednesday since he experienced muscle spasms during his preseason debut last week. "He looked good out there," Bylsma said.
Forward Kyle Okposo did not practice due to a bruised knee. Balsam said he expects Okposo to participate in a morning skate Thursday, but he is unlikely to play in the opener.
Okposo signed a seven-year, $42 million contract with the Sabres in July after eight seasons with the New York Islanders, becoming Buffalo's second-highest-paid player, after O'Reilly, who signed a seven-year, $52 million deal shortly after being acquired in a trade from Colorado last summer.