The San Francisco 49ers fear Jimmy Garoppolo tore the ACL in his left knee.
Garoppolo will have an MRI on Monday to confirm the extent of the injury, but the initial prognosis within the team is not positive.
"Any time you lose your starting quarterback, that's a big deal," Kyle Shanahan said. "It was when we lost our starting running back [Jerick McKinnon] too. So obviously -- I feel for [Garoppolo], personally; I know how disappointed he is. I haven't got a chance to see him yet, but I feel for him."
After gaining 13 yards, Garoppolo had the choice between stepping out of bounds or staying in and taking a hit from a defender. He took an awkward step with his left foot near the sideline, then rammed into Chiefs cornerback Steven Nelson at Kansas City's 7-yard line.