Cardinals running back D.J. Foster takes a hit while flying during last weekend's win over the Jaguars.

Adrian Peterson didn't practice all week because of a neck injury, but the Cardinals will check him out Sunday and haven't ruled the running back out.

"He's played long enough he doesn't need to practice," coach Bruce Arians said Friday.

It makes for an interesting what-if for the Cards in their offensive backfield, however. Kerwynn Williams will play after hurting his ribs against Jacksonville – Arians had just said Monday he might miss time – and Arians called him "tough as nails" after Williams worked through practice limited all week.

He is the likely starter if Peterson can't go.

Then there is D.J. Foster, the undersized third-down back who got a role for good when the Cardinals released Andre Ellington and now who could be thrust in a more prominent part.

"(Carries) could easily fall into his lap," Arians said. "Quickly."

In two games, Foster has one rushing attempt, for four yards. He also has five receptions out of the backfield for 44 yards, including a clutch 12-yarder on the next-to-last play against Jacksonville to set up Phil Dawson's 57-yard game-winning field goal.

[READ: JIM HART, AND FRIDAY BEFORE THE RAMS]

Foster said he wants to move on from that play, but admitted he's been anxious to get time ever since signing off the Patriots' practice squad back in early September.

"In my mind, I wanted to get on the field as soon as possible but I understand the business side of it and that's not always possible," Foster said. "Coming in, I knew I'd have to earn my spot."

The other crucial catch on that field-goal drive was made by Williams, who cracked a couple of ribs in the game but kept going.

"I knew my ribs were hurting," Williams said. "I didn't know to what extent. I was definitely pretty sore. But hey, you have to be out there. And if you're out there, you have no excuse then. You've got to make that play."

The Rams provide a difficult test, even though they are 27th in the league in run defense. In the teams' first meeting in London, Peterson had just 21 yards on 11 carries, and the Cards only ran 12 times total in the blowout loss.

Besides Williams and Foster, the running backs behind Peterson are Elijhaa Penny and just-promoted-from-practice-squad Bronson Hill.

PREP MAKES BYNES VALUABLE

Earlier in the week, Arians said that if Deone Bucannon couldn't play because of his bad ankle, the Cardinals would explore their options, including moving rookie Haason Reddick back inside. But the way veteran Josh

Bynes played last week – 10 tackles, two passes defensed and a forced fumble – it will be Bynes playing for Bucannon, who is out.

The fact Bynes got no physical reps last week in practice in the Cardinals' extensive third-down defensive package and then executed it flawlessly left an imprint.

"He had no mental (errors), was extremely disruptive, and that's not even what he did on first and second down, how disruptive he was in the run game," defensive coordinator James Bettcher said. "It speaks to him as a pro."

The Cardinals monitor how much video players watch each week on their team-issued iPads, and Bynes piles up among the most minutes on the team, Arians said.

"He prepared like he was a starter, and that's what you ask your starters to do," Arians said.

JOHN BROWN, PETERS ARE OUT

Bucannon, wide receiver John Brown (toe) and defensive lineman Corey Peters (ankle) all will be out Sunday, as will rookie safety Rudy Ford (knee). Joining Peterson as questionable are defensive lineman Josh Mauro (ankle) and tight end Troy Niklas (neck/back).

For the Rams, wide receiver Robert Woods (shoulder) and linebacker Connor Barwin (forearm) will not play. Running backs Malcolm Brown (knee) and Lance Dunbar (knee) are both questionable, although both practiced full Sunday.