The Arizona Cardinals' backfield is so depleted that they are set to give a rookie running back his first start.

The St. Louis Rams won the last time these teams met thanks to a rookie running back, who could be getting the ball from a different quarterback this time.

These NFC West rivals are headed in opposite directions, with the Cardinals winners of five straight and the Rams losers of four in a row heading into Sunday's matchup in St. Louis.

Arizona (9-2) can match a six-game run from last season but won't have Chris Johnson available after the club's leading rusher was placed on injured reserve Tuesday. The Cardinals haven't disclosed the injury, though reports indicate Johnson fractured part of his tibia in last Sunday's 19-13 win at San Francisco. He can't return until the Super Bowl.

Andre Ellington, who has started twice, was also hurt last weekend with turf toe and his status is in doubt.

That leaves third-round pick David Johnson as the starter. He has 139 yards rushing, four touchdowns on the ground and three through the air but has fumbled a total of three times.

''David is working on his ball security. He's much better,'' coach Bruce Arians said. ''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 world in him and him starting.''

St. Louis (4-7) is enduring its longest losing streak since dropping the final seven games of a 2-14 campaign in 2011 - the year before Jeff Fisher was hired. This slide has put Fisher's job in jeopardy, though he isn't concerned.

''I've never gone into a game or into a season worried about my job security," Fisher said. ''That would be unfair to the organization, unfair to the players."

Fisher said he hopes Case Keenum can return from a one-game absence due to a concussion. Nick Foles was back as the starter in last Sunday's 31-7 loss at Cincinnati one game after losing the starting job.

Keenum is 2-9 as a starter.

''Hopefully he's going to be back this week and he'll play,'' Fisher said. ''I'm going to give Case a fair shot.''

Foles had his best game of the year in the Rams' 24-22 victory at Arizona on Oct. 4, throwing for three TDs - two to Tavon Austin - with a 126.9 passer rating. That contest marked Todd Gurley's first start, and the rookie ran 19 times for 146 yards.

That was the first of four straight 100-yard efforts for Gurley, who is averaging 54.8 yards in four games since after a 19-yard, nine-carry effort last Sunday.

"Our focus is on improving inside, the interior part of our offensive line and try to get the run game going and keep Todd going," Fisher said.

The Cardinals feature the NFL's fourth-best run defense, yielding 91.1 yards per game. Gurley is the only back to top 100 yards against them.

Captain Patrick Peterson called a defense-only meeting to go over mistakes from last weekend. The Cardinals are trying to stay on track after losing four of six to close 2014.

''We're all reminded of last year,'' Mathieu said. ''We don't want to have that relapse again. It's very important for us to stay focused, continue to find ways to win games.''

They didn't have Carson Palmer during those woes. Palmer did not throw for a touchdown last weekend for the first time, dropping to second in the league with 27.

The Cardinals won 12-6 at St. Louis last season after losing their two previous visits there.

"This will be an NFC West dogfight just like this past week," Arians said.