49ers quarterback Jimmy Garoppolo tore his ACL in Sunday's loss to the Chiefs, the team announced.
Adam Schefter of ESPN.com first reported the news.
On Monday, Garoppolo had an MRI on his left knee, which he injured in the fourth quarter of the 38-27 defeat. As he was running near the sideline, Garoppolo planted his left foot, and it buckled just prior to making contact with cornerback Steven Nelson.
After the game, coach Kyle Shanahan told reporters that the team feared it was a torn ACL.
In February, Garoppolo signed a five-year, $137.5 million deal to stay with the 49ers. San Francisco acquired him from the Patriots in the middle of last season for a second-round pick. New England drafted Garoppolo out of Eastern Illinois with a second-round pick in 2014.
With Garoppolo done for the year, backup C.J. Beathard is likely to take over as the starter going forward, unless the 49ers were to make a trade for another quarterback.
San Francisco is 1-2 this season and will face the 1-2 Chargers in Los Angeles Week 4.