All-Pro left tackle Trent Williams injured his ankle last week against the Broncos. He is out 4-6 weeks with a high-ankle sprain.
Colton McKivitz made his fifth career start Monday night, replacing Williams, but the second-year player didn’t make it through the game.
The 49ers announced in the third quarter that Williams is out with a knee injury.
That leaves Jaylon Moore to play at left tackle. Moore struggled in the seven snaps he played last week against the Broncos.
The Rams have added a field goal in the third quarter, drawing them within 14-9 of the 49ers headed to the fourth quarter.