Getty Images

BOURBONNAIS, Illinois — After the 2013 season, the Minnesota Vikings were looking for a new coach. Jared Allen, whose contract was expiring soon, had something to say about it. "If you hire someone who runs a 3-4 [defense], I'm out of here," he told team executives, with a smile.

Behind the scenes, his agent delivered the same message. And he was as serious as a bull rush. Jared Allen wasn't going to be anybody's outside linebacker.

One year later, you would think the NFL's active sack leader would have become even more set in his ways, more resistant to change, more likely to yell at someone to get off his FieldTurf. This is a player who estimates he has played 99.9 percent of his NFL snaps at right defensive end, and who has done it very well, having been invited to five Pro Bowls.

But a funny thing happened on the way to Canton.

Last season, after signing a free-agent contract with the Bears that called for him to be paid a guaranteed $15.5 million over two years, Allen caught pneumonia and lost 18 pounds in September. He never really recovered during the season and finished the year with a career-low 5.5 sacks. In the preseason previews, he was hailed as the player who would put the Bears over the top. In the postseason reviews, he was just another part of the problem on a 5-11 team.

At season's end, Allen found himself in a place he has not been since he was a rookie in Kansas City—a place where nothing was certain.

In 2015, Allen would be working with his sixth NFL defensive coordinator. But now, it is more like he is working for Vic Fangio. The new Bears coordinator invited Allen to his office shortly after he was hired to tell Allen about his new role.

The first thing to understand, Fangio said, is outside linebacker is the only role for Allen in the new scheme. But that wasn't necessarily a bad thing to Fangio, who then described his vision for Allen.

"I told him he could have a rebirth of his career," Fangio said. "I said it's an easier position to play physically than defensive end. If he dove into it—all in—I thought he would do well as long as he still could run. As long as he wasn't worn out from a physical standpoint, I told him he could do it."

Nam Y. Huh/Associated Press

Allen listened respectfully, knowing about Fangio's history with 3-4 outside linebackers like Rickey Jackson, Kevin Greene, Cornelius Bennett and Aldon Smith. "For a coach who has seen it all to think I can be successful in this defense, that gave me confidence," Allen said.

So he opened his mind and made the decision to go all in.

Outside linebackers coach Clint Hurtt made cutup tapes for Allen to watch. Terrell Suggs, Elvis Dumervil, Justin Houston, Von Miller, DeMarcus Ware—this is how it's done. Allen wanted more. He studied Aldon Smith, Tamba Hali and Julius Peppers.

In the course of his tape work, Allen kept noticing something. Some of these outside linebackers—even the great ones who inspired fear in opponents—were getting occasional one-on-one matchups with tight ends, even running backs. That never happened to him at defensive end. Allen, who already has more sacks than all but eight players in NFL history, was warming to the idea more and more.

But this wasn't just about padding a sack total to Allen. He also saw it as an opportunity to take his run defense to another level, according to Hurtt. And he was intrigued by the challenge of enrolling in Pass Coverage 101—the same course some of his teammates were taking before they started to shave.

Prior to training camp, Hurtt put together a tape that showed the best offseason plays made by linebackers. On it is a play from the second week of OTAs. At that time, Allen still was using training wheels at outside linebacker. He was lined up in the slot on Eddie Royal, the shifty wide receiver who would be the star of offseason workouts. It was the kind of matchup an offensive coordinator would grin wickedly about.

The tape shows Allen getting an excellent reroute as the quick wide receiver came off the snap. Then he drops back into underneath coverage. Quarterback Jay Cutler tries to squeeze the ball to Royal just past Allen, but Allen is in just the right spot, and he uses his length, coordination and anticipation to bat the pass and prevent the completion.

On that play, you couldn't help but notice something. At 6'6", 250 pounds, Allen doesn't look like an outside linebacker. In fact, if you gathered all the outside linebackers in the NFL and took a photo, Allen would look like a 10th-grader in a class of seventh-graders. According to one team's database, he is one of three outside linebackers who are as tall as 6'6". The others are Houston's Lynden Trail and Peppers, who like Allen played defensive end for most of his career before leaving the Bears and signing with the Packers in 2014.

"It makes him unique, and it creates advantages for Jared," Hurtt said. "It makes it difficult for tight ends and offensive tackles to get into him with his length."

Being so tall and playing from a two-point stance as opposed to a three-point stance has forced Allen to be more conscious of pad level.

"Because you are already standing up, you have to make a conscious effort to bend your knees and get low," Allen said. "Especially in the run game. When I'm striking, I have to get down to get back up. Before, out of my three-point stance, I was striking on the rise. Now the blocker has built-in leverage on you."

Those long arms can help prevent blockers from getting to the point where they can use leverage against Allen. Excellent hand use always has been one of his signatures. Now, he believes, it is more important than ever.

Nam Y. Huh/Associated Press

Long players like Allen typically are not athletic enough to do everything required of an outside linebacker. But Hurtt believes Allen is an exception because he is a fluid athlete with good movement skills and "some of the best feet I've ever seen."

A significant adjustment for Allen in his new position has to do with footwork. In his defensive end stance, his inside (left) foot was back. Now it's forward. So now his first step is with his right foot instead of his left, the timing of his counter-rush is off, and the point of when he begins to flip on the edge is different.

Fangio describes Allen's new position as 70 percent defensive end and 30 percent outside linebacker. And it's possible Allen will be 100 percent defensive end if he plays only on passing downs, as the Bears will use four down linemen in nickel in most cases.

But the possibility of him being 30 percent outside linebacker has changed everything. Most of what Allen stored up in his defensive end hard drive has been moved to the trash bin and emptied. It is how it has to be.

"I have to approach this as a totally new position," he said. "I can't try to cross over with what I already know. If I do that, it will cloud everything."

This position switch is more about Allen's mind than his body. You don't have to qualify for Mensa to play defensive end. You read the splits of the offensive linemen and tight end, you look at the backfield formation, and you are off at the snap of the ball.

As an outside linebacker, that's just the starting point. You have to know the coverage and the responsibilities of the other linebackers and defensive backs. You identify the formation. If the formation is trips, you have to recognize the slot receiver. Then, once the ball is snapped, you have to adjust your drop based on receivers' routes.

Allen's eyes have become as important as his feet, and he knows a blank stare can quickly turn into six points for the opponent.

"The biggest thing is understanding route concepts," he said. "That's still a challenge, understanding where to position yourself and who to take. It gives you a newfound respect for linebackers and DBs."

Hurtt said Allen's intelligence has helped him.

"He is aware of where his help is," Hurtt said. "He knows he has the safety up over the top. He will play to his help. Football IQ has been a big help to him making the transition so far successfully."

Allen is smart enough to recognize that releasing his inner outside linebacker has benefits. He feels fresher physically than usual at this point of camp because he isn't bending down on every snap to line up, and he isn't hitting as much.

Nam Y. Huh/Associated Press

He also feels reinvigorated mentally. "You understand now that you can get complacent when you do the same thing year after year after year after year," he said. "Being 33 and having to learn a new position, I have to be on my P's and Q's. This is sparking me to be a student of the game again, to be in my playbook, to learn and focus and really get after it from that standpoint."

The results have been encouraging, but he is a work in progress.

"He's exceeded my expectations in that mentally, it's been very easy for him," Fangio said.

Allen has learned so much in his 11 NFL years. One lesson that has stuck with him is if you can win the battles of reactions, adjustments and countermoves, you can win the game.

He has done that well over time. But he never has had to do it quite like this.

Dan Pompei covers the NFL for Bleacher Report.