Safety Tyrann Mathieu knocks the ball loose from Rams tight end Jared Cook during the Cardinals' game in St. Louis Week One.

It isn't as if Tyrann Mathieu was invisible heading into his first NFL game – indeed, his story was arguably one of the NFL's biggest during training camp – but then the rookie safety made everyone take notice on the field when he raced down the field to amazingly force a fumble from behind by Rams tight end Jared Cook.

Since that touchdown-saving play, Mathieu has been named the NFL's rookie of the month (in October) and moved into the Cardinals' starting lineup. He has played virtually every snap since former free safety starter Rashad Johnson suffered his finger injury the third game of the season.

"Overall, the game slowed down for me a whole lot since my first NFL game," Mathieu said. "Other than that, nothing has really changed. I'm getting comfortable but I've been comfortable with the defense and stepping into the starting role, having to be a leader out there. It makes me have more responsibility, but that's why you play the game."

Defensive coordinator Todd Bowles praised Mathieu's ability to mentally absorb the NFL on the fly – "The game is not too big for him," Bowles said – and emphasized how Mathieu's small size has not been a problem.

"Some guys just know how to play," Bowles said. "He's a small body but a power forward. He can play several positions. I'm not calling him LeBron, but he plays safety, nickel and corner for us. Certain guys just have that knack that you can't teach and he has that."

It doesn't hurt that Mathieu is probably the team's best tackler, a necessity given his stature. He has 62 total tackles on the season, with a sack, two interceptions, a forced fumble, 14 passes defensed and a yearning for better statistics.

"Actually, I don't think I have lived up to my expectations," Mathieu said. "There are a lot of plays out there in previous games that I feel like I left on the field. There is definitely growth that I can make and try to make those plays up these last four games."

PALMER AVAILABILITY IS A GAME-DAY DECISION

Quarterback Carson Palmer, whose injured right elbow kept him limited in practice all week, will be a game-day decision whether he can play, coach Bruce Arians said. Palmer is officially listed as questionable.

"Sometimes it is good this time of year to give the veteran a couple days off and let the young guy get some reps anyway," Arians said. "He got every mental rep. I'm sure he will be fine throwing the ball."

Arians said he had full confidence in backup Drew Stanton if Stanton has to play. The only snaps Palmer has not taken this season have been on specialty plays when running back Andre Ellington took the direct snap.

ELLINGTON, FLOYD READY TO GO

Running back Andre Ellington (knee) "looked just like himself," Arians said, a positive development that should put the rookie back on the field to play Sunday. "He got more and more confident as the week went on." Arians said Ellington isn't even wearing a brace anymore.

Wide receiver Michael Floyd (ankle) said he was at "85 to 90 percent" and he is expected to be fine Sunday.

Tight end Rob Housler, who had been on the injury list with an ankle issue but practiced full Thursday, was changed back to limited Friday with a hip problem and is now listed as questionable. Aside from Palmer, the rest of the dinged Cardinals are all listed as probable.

For the Rams, tackle Jake Long (concussion) is probable. Starting center Scott Wells (fibula) is out, while cornerback Brandon McGee (foot) is questionable.

STACY CHANGES GAME FOR RAMS

The Rams had some issues running the football early in the season, but coach Jeff Fisher said after the team's mini-bye following a Thursday night loss against the 49ers, St. Louis regrouped. By "regrouped," it meant the Rams put rookie Zac Stacy in the lineup.

Up until that point, Stacy had just one carry for four yards on the season, in the opener against the Cardinals. In the fifth game of the year, Stacy got 78 yards on 14 carries, and it went from there. Stacy has 696 yards rushing and is averaging 4.4 yards a carry in basically eight games of work.

"That's helped our cause," Fisher said.