ALLEN PARK -- Detroit Lions cornerback Rashean Mathis is an old dog who is learning a few new tricks.

Among them: Nickel back.

Lions defensive back Rashean Mathis could shift from corner to nickel against the Bears.

Regular nickel man Bill Bentley appears unlikely to play Sunday against the Chcago Bears, after sustaining a knee injury Oct. 27 against Dallas. He hasn't practiced this week.

That means Mathis, 33, likely will slide from corner to nickel when Detroit wants to go with an extra defensive back, and rookie Darius Slay will take Mathis' spot at corner.

And Mathis says he's fine with that, if called upon, after getting some practice time at nickel this week.

"I'm very comfortable," he said. "Practice helps a little, but in the heat of the game, it's just going in the memory bank, going into the tool belt, and pulling out tools to make plays.

"I have the tools in my tool belt. I had to knock off rust of some of them, but I still possess them."

Mathis, who played some nickel in the past with Jacksonville, said the biggest difference between nickel and corner is that nickel requires more patience. The receiver has more acreage to roam when lined up in the slot.

The receiver also can cut in either direction, whereas outside receivers often lean one way or the other. And that takes some getting used to.

"When you're out there on the outside, their alignment dictates whether they're going inside or outside," Mathis said. "But in the nickel, you never know. You just have to be solid and patient -- a little more patient in the nickel than in the outside. And I think I possess a little patience.

"They say you can't teach an old dog new tricks. But I feel you're never too old to run."

Mathis didn't sign with Detroit until midway through training camp, but has turned out to be a significant addition. He originally entered the lineup in Week 3 after Slay struggled at right cornerback, and has started every game since.

But Slay has shown noticeable development as well.

When Bentley left the Dallas game, Mathis slid to nickel and Slay entered at corner -- a preview of the look likely to be deployed against Chicago -- and the rookie held his own.

But he also played against the Bears earlier this year, and struggled to handle their wideouts' size. Alshon Jeffery, who stands 6-foot-3 and 216 pounds, caught a 14-yard touchdown pass over Slay.

Slay says he's now better prepared for the job.

"They used their size against me most of the night," Slay said. "The one deep ball they got in the end zone, he just out-stretched me there. I was in perfect position there, but his strong hands and his big body and big frame, I couldn't get the ball.

"There's a whole big difference now. I'm ready. I'm basically going to try to get the ball "before they get it, and that's the difference."