But considering he was the team's third-leading tackler last year, plays as physically as anyone on the team and really came into his own in the playoffs, not having safety Atari Bigby in the lineup or at full strength has to have had some effect.

Bigby seemed poised for a big season coming out of training camp, but one sprint into the end zone at Ford Field flipped his world upside down. Suffering a severe hamstring pull, Bigby missed the next five games, and upon his return after the bye week injured an ankle. That, combined with the residual effect of the hamstring injury, left him a fraction of the player he was to start the season.

"I'm not 100%," Bigby said. "I'm not all the way back. I can't even give you a gauge on it. The hamstring, it just affects your burst, trying to push off, being explosive. I'm an explosive player; that's my game."

Against Tennessee, Bigby ran down running back LenDale White from behind on his 54-yard run, but that was just a test of his open-field speed. It wasn't a barometer on how much confidence he has in his body or his ability to change direction quickly or close on a ball carrier.

Against the Minnesota Vikings Sunday at the Metrodome, he started again but was taken out for several series in favor of Aaron Rouse. The move was made to protect his ankle from taking too much of a pounding on the artificial turf, according to secondary coach Kurt Schottenheimer, as well as to give Rouse an opportunity to play.

When Bigby will be at full strength isn't clear. Although the hamstring is healed, it doesn't mean it is as strong or as flexible as it was before. The coaches said he showed marked improvement between the Tennessee and Minnesota games, but the proof will be in his stride Sunday against Chicago.

"It's one of the most frustrating (injuries) because it affects you running, affects your burst, how you open up, how you play naturally," said running back Ryan Grant, who fought through the same injury earlier this season. "I see it a little bit with Bigby. I know he's trying to do the best he can and get it right, but it's definitely frustrating."

The argument that Bigby's reduced role in the defense is the cause for so many big rushing plays can be summed up in his play against run-oriented division opponents last season. In those six games, he had 25 solo tackles (37 total), two interceptions, five passes broken up and two forced fumbles.

Often lined up closer to the line of scrimmage than fellow safety Nick Collins, Bigby was a clean-up guy who could serve as an eighth defender in the box or as an extra line of defense, there to prevent a running back from turning a 12-yard gain into a 50-yard gain.

Last season, with Bigby in the lineup all 16 games, the Packers allowed seven runs of 20 or more yards, two of them longer than 40. In nine games this season, the defense has already allowed 10 gains of 20 or more yards, including three beyond 40.

"I think he's played a big role in our run defense," linebacker Brady Poppinga said. "Essentially, he's our last line of defense. A lot of times, he's the extra guy in the box when we're trying to create a run defense presence."

Poppinga, however, does not buy that the woes defending the run can be pinned on one particular position or the absence of one player. He maintains that it has been the result of various players making mistakes or not getting the job done at inopportune times.

Still, Bigby ranked third on the team in tackles last season with 121 - and second in solos with 95 - and there isn't anyone picking up that slack. Collins has been playing the best Football of his career and has increased his number of tackles from last year by more than two per game, but he mostly makes his living playing deep.

In his two games since returning, Bigby had 15 tackles, but few that were memorable. He had a shot at keeping Vikings running back Adrian Peterson out of the end zone on the eventual winning touchdown run, but he was lined up wrong because of a miscommunication and had a poor angle on Peterson.

Bigby did manage to hit him as he got to the goal line.

"I think it started to come back more last week than it did the first week," Schottenheimer said of Bigby's natural talents. "You can only imagine, when you're used to doing something at full speed and 100%, where you see things you recognize and know the angles you have to take and the drive you have to put on it.

"When your body's not at 100%, that's like a new learning experience. You may have to adjust a little bit. But those issues were a couple weeks ago. I think he's made progress. Hopefully, there will be more this week."

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