Jacobs was injured when he was hit by Kareem Jackson at the end of an 8-yard run late in the first quarter. Jacobs remained on the turf for several minutes while trainers talked with him. He was helped to his feet and hopped off the field on his right leg before being placed on a cart and taken to the locker room.
Jacobs had X-rays, which were negative on the knee. He will undergo an MRI on Sunday, a league source told ESPN NFL Insider Adam Schefter.
Jacobs joined the 49ers in the offseason after seven seasons with the New York Giants.