ALLEN PARK -- Tahir Whitehead has flourished into a special teams ace since the Detroit Lions took him in the fifth round of the 2012 NFL draft. He was the team's special teams player of the year in 2013.

But the linebacker has yet to play a single snap of defense. And that could be about to change.

New coach Jim Caldwell has moved Whitehead from left outside linebacker into the middle. And with starting middle linebacker Stephen Tulloch excused for the final two days of minicamp last week, it was Whitehead who got the call to run with the ones.

"He's an explosive guy," Caldwell said. "He's got punch and can run. Certainly, he can direct traffic in there. He's got a real good feel for things. He's a good football player. You see him close ground out there on the field. The guy can make plays.

"Obviously, Tully's a little bit bigger than he is in terms of weight and girth -- not in terms of height. Nevertheless, I still think that he can function in there."

Tulloch, a captain last season, will continue to start in the middle, flanked by DeAndre Levy on the weak side and likely rookie Kyle Van Noy on the strong side. But Whitehead appears to be ascendant among the reserves fighting for the fourth linebacker role, a competition that also includes Ashlee Palmer and Travis Lewis.

"Only time will tell," Whitehead said. "I just got to continue to work at it, continue to hone my craft."

Whitehead, at 6-foot-2, weighed about 230 pounds when the Lions selected him out of Temple two years ago. He was a bean pole, as far as linebackers go. And that's a big reason why he was not a viable candidate to play defensively.

But Whitehead has packed on the pounds this offseason, now weighing around 240, in preparations for his move into the middle.

The next step is digesting the mental hurdles of such a move.

"What's hurt me in the past is my keys," Whitehead said. "I'm trying to train my eyes to get to where I need to get to as quickly as possible. See where the ball is, and track it down.

"In 11-on-11, it gets hard because your eyes start to go all over the place. You may lose track of who you're supposed to be looking at."

To that point, Whitehead has hit the film hard this offseason to learn his new keys. He's taken to actively watching the film -- getting out of his seat and going through his steps as though he were actually in the game.

With Tulloch out last week, Whitehead got a chance to test out some of those lessons, an important step in his evolution.

"It's been amazing," he said, "helping me learn the defense and be able to play different roles and go wherever they need me.

"The keys are the difference. You're right in the middle of it all. There's a lot of communication, because you have to direct traffic. Communicate to the D-line, the safeties and everything. But I was able to get comfortable in there and just do what I was supposed to do."