Allen Park — Despite losing Stephen Tulloch, the middle linebacker and defensive captain the past two years, the Lions defense has actually improved the past three games.

Between the continued improvement of DeAndre Levy, a weak-side linebacker who spent two years in the middle, and the strong play of Tahir Whitehead, who's filling Tulloch's spot in the middle, the Lions linebackers have played key roles in the defense ranking No. 1 in yards and points allowed, No. 1 in pass defense and No. 2 in rush defense.

The Lions also have had significant contributions from Ashlee Palmer on the strong side and Josh Bynes in spot duty last week, and the unit could improve even more when second-round pick Kyle Van Noy is activated from injured reserve with designation to return, likely in Week 10.

But because the Lions defense predominantly uses its nickel package, Levy and Whitehead are the ones who always patrol the second level.

Levy is 235 pounds and Whitehead is 242, so neither is especially big. And even though they've both proved to be sure tacklers, neither has many punishing hits. Instead, they both rely on speed and instincts to rack up tackles.

"With the amount of passing and the spread and trying to spread people out, you have to have linebackers that can run," defensive coordinator Teryl Austin said. "But you also have to have physical guys. You have to have guys that know how to play football. I think sometimes that gets overlooked."

With Levy, that's not the case. Whitehead said Levy has taken on a bigger leadership role to help fill the void left by Tulloch, who tore the anterior cruciate ligament in his left knee in Week 3.

Levy leads the Lions in tackles with 57 and is tied for fifth in the NFL.

"He understands the game," Whitehead said. "He understands how offenses are trying to attack our defense, so he's able to play a lot faster because he puts in a lot of work during the week with the film. There's nothing really in particular. He's a smart player, he's a fast player."

Levy does spend extra time in the film room each week, but he's already in his sixth season as a starter, which has helped him develop natural instincts, which Austin said are "off the charts."

"You see him key in and diagnose a play, and that's why he gets there so fast," Austin said. "That's why he makes a lot of tackles. It's not just because of his speed, it's because he is able to key and diagnose really fast and he can get there."

Even though Levy had to adjust to a new defensive scheme, there hasn't been any fall-off from his career year in 2013 when he had 119 tackles, eight for loss, 15 passes defenses and six interceptions. Levy has one interception, three passes defensed and seven tackles for loss this year.

"Instinctive players have a way of showing up consistently and he does," coach Jim Caldwell said. "He works so hard. We sit in the meeting rooms, and I sit there and I listen. He asks very good questions, questions that not only serve him, but the rest of the room, as well. He's one of those guys that without question could probably teach his position."

Whitehead didn't play a defensive snap until his third season, but earned the starting strong-side linebacker job out of training camp this year. When Tulloch suffered the injury, Whitehead moved to the middle and has been productive with 35 tackles, two for loss and two interceptions, both of which came in Week 6 against the Minnesota Vikings.

"I ask Levy questions; I ask Tully questions," Whitehead said. "I try to pick their brain to be able to play like they play because they're both great players."