Coming into the 2009 season, the Packers felt relatively secure about the number of options available at linebacker as they made the transition to Dom Capers' 3-4 defensive scheme.

Turns out, it's a good thing they had all those choices.

Because to look at the linebacking corps now with four games left in the regular season, it's significantly different in terms of roles and playing time from the one the Packers began the year with.

Consider where the unit was when the season opened:

--At the inside linebacker spots, Nick Barnett was a part-time player as he came back from his 2008 knee injury, A.J. Hawk played in the base defense only, and Brandon Chillar was a regular in the nickel package and "Big Okie" five-linebacker set.

--On the outside, Aaron Kampman was learning his new position and the other flank saw Brady Poppinga starting, rookie Clay Matthews on third downs, and Jeremy Thompson in waiting if needed.

But now? Fast forward 12 games and it's an entirely different story for a unit that has evolved as the entire defense has risen to No. 1 in the league in yards allowed.

Barnett has been playing full-time since Week 4, while Chillar broke his hand in Week 8 and has taken very few snaps from scrimmage since. Instead, Hawk has become a full-time player, working in both the base and nickel defenses.

Meanwhile, Matthews took over a starting and full-time role in Week 6, and Kampman went down with a season-ending knee injury in Week 11. Rookie Brad Jones, who missed a good portion of training camp with back troubles and was on the roster bubble for a while during the preseason, has assumed the other starting role, with Poppinga now in reserve and Thompson suffering a neck injury that has possibly ended his season.

A full-time quartet of Barnett, Hawk, Matthews and Jones is not necessarily the linebacker group anyone expected would be saddled with keeping the defense at the top of the league and making a playoff push, but that's where the Packers are right now. Most important, for all the adjustments and changes along the way, Capers likes what he has seen.

"I think the thing that it points out is you have to have depth," Capers said. "It's a long season. You're going to go through a lot of things - injuries, different packages you want to use.

"We've been fortunate because we've had guys that have stepped in and done a good job for us."

Here's a look at the current quartet seeing the field the most and what they've brought of late to the defense.

BARNETT

Even though he wasn't entirely happy about it at the time, Barnett admitted around midseason that he benefited from the "snap count" he was on during the first three weeks. By the time he played a full game for the first time, at Minnesota on Oct. 5, he was four days shy of 11 full months since his knee injury - which occurred in Minnesota, coincidentally - and fully capable of making it through the long grind of the season.

Barnett's impact has been undeniable since returning to full-time duty. He posted his season high of 13 tackles in that first Minnesota game, and he's had three other double-digit tackle games since.

All three of his sacks have come since the Week 5 bye, including two in the defense's best overall effort of the season vs. Dallas in Week 10. He's also posted six additional tackles for loss over the last seven games, most notably a stuff of Baltimore running back Ray Rice 7 yards behind the line of scrimmage on a swing pass/lateral this past Monday night.

"He's gotten a lot more comfortable being the signal caller," Capers said. "I think he's given us good leadership. One thing about Nick, if there's something that's not clear in his mind, he's going to come and ask. I like that. He's got a lot of pride in doing things right."

Barnett is listed on the injury report this week with a knee strain, but after sitting out on Wednesday, he returned to practice Thursday in a limited fashion and was a full participant on Friday. He is probable for Sunday's game in Chicago.

"He'll throw his body around, and he plays with a sense of relentlessness," Capers said. "I think he's made a lot of progress in terms of the defense and understanding his gap fits and all of those things."

HAWK

Hawk's snaps seemed few and far between at times early in the season, with Capers using so much nickel and Hawk playing only in base.

But since he had two big hits to stop running back Jamal Lewis for losses at Cleveland in Week 7, his game has definitely picked up. So much so that when Chillar broke his hand in the second game against the Vikings, the coaching staff turned over the nickel role to Hawk and he hasn't disappointed.

He got his first sack of the season at Tampa Bay, the first game Chillar missed, and then this past week against Baltimore he turned in one of his best games. Chillar was back to playing from scrimmage, but only in the "Big Okie" package, while Hawk led the team with seven tackles, even missing a couple of others he should have had.

Most impressively, in the fourth quarter he ran stride-for-stride with tight end Todd Heap down the middle seam and turned his body at the perfect time to make an athletic interception, almost identical to one he had his rookie season of 2006 at San Francisco against tight end Vernon Davis.

"His interception was a huge play in the game, and I think it was an excellent play by an inside linebacker," Head Coach Mike McCarthy said. "A.J. is tough, he's very consistent.

"He's the same guy every day. He's very coachable. He's always working to get better, and I think he's put together some very solid days for us."

MATTHEWS

McCarthy has used similar words to describe Matthews - consistent, steady and coachable - and the rookie's playmaking ability is just what the Packers were looking for when they traded up to get him in the first round of the draft at No. 26 overall.

{sportsad300}Matthews leads the team with seven sacks and three fumble recoveries, and he had already won "Pepsi NFL Rookie of the Week" twice before winning NFC Defensive Player of the Week for his performance against Baltimore, which featured his second multi-sack game and a career-high six tackles.

Matthews is a player who has only gotten better with increased playing time, beginning with the shift into the base defense in Week 6. He's become the every-down threat at outside linebacker the Packers were hoping for, and one they've needed even more with Kampman done for the season.

He plays with a motor that never stops, as evidenced by his sack on Thanksgiving Day when he chased Detroit quarterback Matthew Stafford in three different directions before burying him on the final play of the first half.

"I just hope he can still continue to improve every week, because he has," Capers said. "You've seen him have some big games, but the thing about Clay is he's not a one-dimensional guy. He's physical against the run. He's got the quickness to rush, and I think he's gaining confidence and rushing better all the time. And he can drop (into coverage). Those are the kind of guys you like to have out there."

JONES

A seventh-round draft pick out of Colorado, Jones was basically absent in training camp with his back injury until a sack-fumble in the second preseason game against Buffalo opened some eyes. He began the regular season by contributing on special teams, and then with little to no time from scrimmage was thrown in as the starter against Dallas in Week 10 when Kampman was out with a concussion.

He performed well enough that the coaching staff didn't hesitate to turn the job over to him when Kampman hurt his knee. Jones has said at various times that he gains more knowledge and comfort with each game he plays, and Capers has seen that progress.

"There's something about having to start on the bottom rung and fight for every step of the way," Capers said. "We saw some things in training camp that we liked. You never know, the guy is a late-round draft pick, but you hope you see some redeeming qualities, and then you take and build on that."

Jones' growth continued as he posted his first NFL sack this past Monday against Baltimore, and for a young guy who in some ways came out of nowhere, he's channeling his enthusiasm and energy as best he can.

"Anytime you're part of a defense like this, it's amazing," he said. "I'm just excited to be a part of it and have a contributing factor in it."

There's no telling if the evolution of this position for 2009 is complete, but the fact that the unit has been able to change and adjust so much speaks to the quality of the group that was put together at the start.

Who knows whether the current four will be the final four during a potential playoff run, but either way, Capers plans to be prepared.

"It just points out that you have to constantly be thinking in terms of depth and what you're going to do at every position if you lose any one player," he said.

"I like our linebacker corps, I've liked them from the beginning. I think we've had good depth. I think we have versatility there where guys can play different positions, and it gives you flexibility in your defense."