Garoppolo appeared to sustain the injury prior to the collision.
The 49ers can ill-afford another injury at running back, but starting running back Matt Breida went down with an apparent knee injury late in the first half against the Chiefs. That would knock him out for the rest of the season.
After gaining 13 yards, Garoppolo had the choice between stepping out of bounds or staying in and taking a hit from a defender. His left knee buckled as he planted his foot trying to cut upfield, then was met by Nelson.
San Francisco 49ers quarterback and former Patriots backup Jimmy Garoppolo left Sunday's game against the Kansas City Chiefs with what the team fears is a serious knee injury. Garoppolo is a healthy 7-2 against the spread in his career starts, but that "2" has come in the past two weeks. After a visit to the medical tent, he was carted off the field. If the injury is the worst-case scenario, it's a awful blow for a 49ers team that had high hopes coming into the season. He signed a five-year $137 million deal in the offseason and was one of the NFL's bright young stars.