The defensive star was helped off the field and carted to the locker room when the incident took place. An MRI on Monday revealed the injury.
This is is the second major injury he has suffered with the Niners, as he also blew out his knee in the 2013 NFC Championship Game. He has been a key part of the Niners' success, which includes three straight NFC title appearances, and one Super Bowl trip.
His contribution lead to a four-year contract extension this past summer worth $44 million. This keeps him in the Bay Area through 2022.
In other notable news around the league, the Colts have released cornerback Antonio Cromartie, whom they signed this past August.
Cromartie started all four games this season, but struggled in every opportunity. It all capped off with a horrible series in London, which saw him get called for two penalties, and burned by a Jaguars receiver.
Indianapolis defensive struggles have lead to their 1-3 start, as they rank 24th in the league with 328.8 yards allowed per game. From that stat alone, they knew something had to change.
In another surprising roster move, the Baltimore Ravens have cut running back Justin Forsett after benching him last week. The move was described as "mutual" after Forsett was inactive in Sunday's loss to the Raiders.
The Ravens have yet to make the move official, but this reportedly came from the 30-year-old expressing his desire to find another team. The veteran averaged just 3.2 yards per carry in the first three weeks, which tied a career low.