GREEN BAY, Wis. - Clay Matthews understood why everyone was so concerned.

But the Green Bay Packers five-time Pro Bowl linebacker Clay Matthews insisted that missing Monday morning's practice with what coach Mike McCarthy termed "knee soreness" is not cause for alarm.

"I appreciate you guys' worry," Matthews said in a brief post-practice appearance in the locker room. "Honestly, I'm going to be fine. You'll be like, 'Oh, Clay just gave us a heart attack.'"

Packers coach Mike McCarthy also downplayed the injury, saying the team was "just being smart with him."

Given Matthews' injury history and his importance to the Packers' defence, any injury that would keep him off the field once the regular season begins Sept. 13 would be a major blow to a unit that turned around when the coaches began using him at inside linebacker in addition to his work on the edge.

At the time of the move, the defence was ranked 25th in the 32-team NFL, including dead last against the run. By season's end, Green Bay was 15th in overall defence and No. 23 against the run, as the Packers went from giving up an average of 153.5 yards per game on the ground during the first eight games to 86.4 yards per game in the final eight.

The team went into the off-season intent on playing Matthews inside more, choosing not to add another veteran inside linebacker despite cutting the two opening-day starters from last season (A.J. Hawk and Brad Jones) and letting a third part-time starter (Jamari Lattimore) depart as a free agent. The Packers did select Michigan inside linebacker Jake Ryan in the fourth round in May, but Matthews had been working inside extensively before missing Monday's practice.

"(His) workload hasn't changed. He's always worked this much. Where he is working has changed," McCarthy said last week when asked about how Matthews was being used in practice. "He's spending a lot more time at the inside linebacker position. We all understand that. I mean, he's played his whole career at outside linebacker, so the ability to keep moving him over there every third day or so is the plan. Just to get the reps from inside and get the full training because he's never had that from starting in training and going all the way through training camp."

Matthews watched a brief portion of practice wearing a Packers baseball cap before jogging off the field and into the Don Hutson Center, where McCarthy said Matthews did rehabilitation work with other injured players during practice.

Asked how long he might be out, Matthews replied, "I don't know. I have no idea. (We're) just taking the car into the pit shop, getting the wheels changed, getting ready for the season."

Matthews played in all 18 games last season (including playoffs), although he did battle a groin injury for part of the year. He missed a total of six games in 2013, including the Packers' NFC wild-card playoff loss to the 49ers, after twice breaking his thumb.

He has also fought hamstring problems since the Packers took him No. 26 overall in the 2009 draft; Matthews missed one game in 2010 and four games in 2012. The only other game he missed before the thumb injury was the 2011 regular-season finale, when McCarthy rested him and several other key veterans with the No. 1 playoff seed already sewn up.

Matthews felt something in his knee during Saturday's first in-pads practice of training camp, McCarthy said. The players had Sunday off before returning to practice Monday.

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