CHICAGO -- The whereabouts of Chicago Bears middle linebacker Brian Urlacher continue to be a mystery after the eight-time Pro Bowler sat out six straight practices and missed the team's preseason opener against the Denver Broncos on Thursday night.

Urlacher missed four straight days of practice because of knee soreness before leaving the team on Monday and Tuesday for what Bears coach Lovie Smith described as "personal issues." The Bears held a walk-through practice on Wednesday that was closed to the media.

"Brian is doing well," Bears general manager Phil Emery said in a pregame interview with WBBM-AM 780. "We treat each and every injury on an individual basis. He's a guy who needs some rest right now. We're going to provide him with that. Practice right now is not in his best interest or the best interest of the Chicago Bears."

Six-year NFL veteran Nick Roach is expected to start at middle linebacker in Urlacher's place against Denver.

"Everything is OK [with Urlacher]," Smith told WBBM-AM 780 before the game. "He's still away for personal reasons. He'll be back soon."

Urlacher sprained the medial collateral and posterior cruciate ligaments in his left knee in the 2011 regular-season finale. It's unclear if the soreness Urlacher has experienced in his knee recently is directly related to his absences from the practice field this week.