ALLEN PARK -- Detroit Lions cornerback Darius Slay hurt his knee during practice Thursday and was undergoing testing Friday to determine the severity of the injury.

Coach Jim Schwartz said the rookie hurt himself in a "freak thing" during the workout.

Darius Slay takes the field for his breakout game last week against Green Bay. His status going forward, however, is uncertain.

While the club has not announced the severity of the injury, players are preparing as though Slay could miss some or all of Detroit's playoff chase.

That includes Sunday's game against the Eagles and their red-hot quarterback, Nick Foles. Chris Houston and Rashean Mathis are expected to be the starters, while little-used reserve Jonte Green could be their backup.

Mathis said he's spoken to Slay and reminded him that although Detroit will move on without him, he's still their brother.

"Guys gotta step up. Darius has gotta know he's still our brother, but this thing still goes on, and the next guy has to be ready," Mathis said. "Just letting him know he can bounce back from this stronger than ever. Telling him, 'who knows how long it'll hold you out, but just know it's a positive when you ended things.' And that's something he can take, that he was on the incline when he ended things.

"I think that's something he he needs to know -- and not drop your head."

Slay, 22, underwent arthroscopic knee surgery this summer for a torn meniscus. It's unknown whether this injury is related to that one.

The rookie out of Mississippi State began this season as the starter at right cornerback, but lost the position to Mathis after two games. Mathis proceeded to take on a mentorship role with Slay, and has helped him develop this season.

Slay put together easily his best game as a pro last week on Thanksgiving against the Packers, allowing only three completions. He didn't allow a big play until Green Bay's final, meaningless possession.

Days later, his season is imperiled.

That would seem like bad timing, but Mathis said the trick is to use it as a good thing.

"You definitely will hang your head a little about it, but in the grand scheme of things, he's a rookie, and you have to look at it as you have to find a positive -- that you were playing well," Mathis said. "It's not like you were down in the dumps and then you got injured. You were on the incline, and things happen in this league. It's just about how you react to adversity.

"That's something I'm going to put in his memory bank, is you ended it on a good note. And you can build on that mentally."

Mathis is a good resource for Slay not only because he's mentored the rookie throughout the season, but he's also suffered a significant knee injury.

He tore his ACL late in the 2011 season with Jacksonville, and has said the injury impacted his performance throughout 2012. He wasn't retained this year by the Jags, which is how he ended up in Detroit.

Mathis is imparting that experience on Slay.

"He's in good spirits -- as good as he can be," Mathis said. "You just need to let him know we still see him as a brother, and he's still with us. Just do everything you need to do to get back on the field, and we'll be waiting for you."