Mark Zuckerberg has torn one of his anterior cruciate ligaments (ACLs) while training for a fight.
The mixed martial arts (MMA) enthusiast posted a picture on Instagram showing him in a hospital bed with his left leg elevated and heavily bandaged.
The ACL is one of the ligaments inside the knee and "gives the knee joint stability", according to the NHS.
"Tore my ACL sparring and just got out of surgery to replace it," the Facebook co-founder, 39, said.
"I was training for a competitive MMA fight early next year, but now that's delayed a bit.
"Grateful for the doctors and team taking care of me."
Zuckerberg said he was "still looking forward" to his MMA fight "after I recover", adding: "Thanks to everyone for the love and support."
In May, he completed his first jiujitsu tournament.
A few weeks ago, he shared a close-up of his face showing bruising on the bridge of his nose and under his eyes.
It was down to sparring that got "a little out of hand", he said.
In August, Zuckerberg accused Elon Musk of not being serious about the pair having a cage fight, and said it is time to "move on" from the saga.
The chief executive of Meta Platforms said his tech billionaire counterpart did not appear to want to go through with the highly-anticipated clash after failing to even agree on a date.
There were months of back-and-forth between the pair after Musk suggested the idea - only for Zuckerberg to unexpectedly respond that he was prepared to battle his rival.
But the Tesla chief executive said he may have needed surgery before the fight could happen.