Tom Croft, the England and Leicester flanker, has been ruled out for the rest of the season - just one game into the new campaign.
The 27-year-old, who featured for the Lions during the summer, was injured during the Tigers' Premiership victory over Worcester on Sunday.
The extent of the injury only became clear on Monday, and Leicester director of rugby Richard Cockerill has confirmed Croft underwent surgery yesterday.
"Tom had a bang on the knee at the weekend," said Cockerill. "We felt it was relatively innocuous but after further investigation the ACL (anterior cruciate) ligament was ruptured and he has had surgery to reconstruct it and that’s his season finished.
"It’s horrendous bad luck for Tom because it is only last year that he came back from a very bad neck injury and did so well for England but the Lions as well.
"But these things happen. Tom will make a full recovery but it’s his season over. It’s a blow but we have just got to get on with things."
Croft, who has made 36 appearances for England, missed much of last season with a neck injury.