The biggest question about the Lions’ defense three weeks ago involved the cornerbacks. 

After three games, Darius Slay and Rashean Mathis have stepped up. There’s work to do, but they’re off to a solid start.

After three games the Lions’ defense is ranked first in the NFL allowing just 244.3 yards per game. The pass defense is third in the league allowing 180.7 yards per game.

The leadership of the 34-year-old Mathis cannot be overlooked.

“One of the things that is very difficult for individuals to notice is the impact that Mathis has on games. He’s a real settling force for those guys out there,’’ Caldwell said after the win over the Packers.

It’s not just Mathis’ play on the field that Caldwell likes.

“During the course of the game, you can see him communicating back and forth, making certain everything is in place, but what he does in practice and the meeting rooms, he’s absolutely invaluable,’’ Caldwell said.

Slay, in his second season, has shown great improvement over his rookie year.

“He just keeps getting better every week. He gets challenged every week and sees different styles of receivers with different strengths every week,’’ Caldwell said. “He has certainly been able to progress.’’

Thanks to injuries the Lions have used four nickels. The first two — Bill Bentley and Nevin Lawson — are gone for the season. Don Carey injured his hamstring again early on Sunday although the severity is not known. Danny Gorrer, who signed last week, stepped in against the Packers.

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