Garoppolo will undergo an MRI on Monday to confirm the severity of the injury.
Scrambling down the sideline in an effort to pick up extra yardage, Garoppolo watched his left knee buckle just before he absorbed a big hit from cornerback Steven Nelson. He was ultimately carted to the locker room, after NFL Network's James Palmer captured the visibly distraught quarterback on the sideline.
Prior to the injury, Garoppolo had completed 20 of 30 passes for 251 yards and two touchdown passes. The numbers are a little misleading, however, as he started slowly for the third consecutive week, holding the ball too long in the pocket.
In three games this season, Garappolo has completed 59.6 percent of his passes at 8.1 yards per attempt for a passer rating of 90.0.
Although defenses have confused Garoppolo at times in the early going, his loss can't be overstated. Before he took the reins in a furious December flourish, the wayward 49ers were 1-10 under C.J. Beathard and Brian Hoyer. As optimistic as Shanahan may be about Beathard's prospects as a high-end NFL backup, this turn of events has the potential to sabotage the season for an outfit sporting a 1-2 record in a loaded NFC.
The mood in San Francisco's locker room was understandably downtrodden in the wake of Garoppolo's injury.