Arizona Cardinals running back Chris Johnson is reportedly expected to miss six to eight weeks after suffering a fractured tibia on Sunday in a win over the San Francisco 49ers, according to Mike Jurecki of Fox Sports 910.

Johnson's injury was originally believed to be a bruised knee, but on Monday evening reports came out that the injury was more significant than originally thought, and that he suffered a fractured tibia and would be out for some time, according to Fox Sports' Jay Glazer.

The Cardinals have sole control of first place in the NFC West, and with their win over the 49ers, they improved to 9-2 on the season. Arizona looks to be one of the front-runners in the NFC playoff picture, but losing Johnson is a huge blow as he is its lead running back.

Johnson has reemerged as one of the top running backs in the league this season, and was on pace to rush for over 1,000 yards. The Cardinals running back has 814 yards on the season and has scored three touchdowns. Johnson's injury isn't the only hit the Cardinals' backfield took this weekend either, as running back Andre Ellington is dealing with turf toe, and it is unclear if he will be able to play this weekend against the St. Louis Rams.

With Johnson out for some time and Ellington questionable to play, rookie running back David Johnson will step into the starting role. The rookie rushed for 21 yards and a touchdown last week, and will get plenty of opportunities against the Rams this Sunday.

"David is working on his ball security. He's much better," said head coach Bruce Arians, via Fox Sports. "He still struggles sometimes out in space, reading hots and blitzes, like most rookies do, but he did a great job on that goal line by putting his foot into the ground and making that run for the touchdown. I have all the confidence in the world in him and him starting."

The Cardinals will look to create more separation in the NFC West with a win over St. Louis, and despite the loss of Johnson and possibly Ellington for this upcoming week the Cardinals will look to extend their five-game winning streak.