With Atari Bigby on the Physically Unable to Perform (PUP) list, rookie safety Morgan Burnett already has a starting job locked down in the Packers defensive backfield.

Add to that cornerback Al Harris' unavailability (also on the PUP list) and undrafted rookie cornerback Sam Shields also figures to play a prominent role as the nickel cornerback in the Packers' season opening game at the Philadelphia Eagles on Sunday.

"We’re young and we have some experience being young,"  said head coach Mike McCarthy. "You just really focus on getting them ready. Morgan Burnett has done a very good job in his time here, and I feel he is ready to go. Sam Shields has done a very good job. He has been getting better in every practice and he needs to play. We have played with rookies before and we have played with young players before."

The inexperienced duo will be part of a secondary that will have its hands full with one of the best receiving corps in the NFL in DeSean Jackson and Jeremy Maclin.

But as McCarthy said, the Packers are used to being green having been the NFL's youngest team four consecutive years coming into this season. They're used to preparing unproven players to be contributing members of the team.

Sam Shields said he seeks the counsel of defending NFL Defensive Player of the Year Charles Woodson frequently. Meanwhile, Pro Bowl safety Nick Collins said the rest of the defense is out there to help and support their young counterparts.

"You just got to trust them to go out there and play," said Collins. "We've got a veteran group. We've got a lot of veteran guys that are going to be out there with them."

If there's any sense of apprehension or nervousness on the part of the untested Shields, he's not showing it. He confirmed his teammates' confidence in him.

"I know they're confident in me," said Shields. "They're my teammates, they're 100% behind me. Anything I need help on, they're going to help me. They're going to make sure I'm not out there lost."

One has to figure the rookies will be tested by Eagles quarterback Kevin Kolb. Seeing as Woodson and Collins have 16 and 13 interceptions respectively over the past two seasons, it only makes sense that Kolb won't be testing them very often.

With the choice between throwing in the direction of two ball-hawking Pro Bowlers or raw rookies, the rookies can expect to be tested early and often.

In an interview with Green Bay media on Wednesday, Eagles head coach Andy Reid joked about targeting Shields.

"I'm just an okay offensive guy, but I'm going to look at that now that you told me," quipped Reid.

The job faced by Shields and Burnett will be all the more difficult as they have to deal with a receiving corps that includes Jackson and Maclin, not to mention Jason Avant and tight end Brent Celek.

Jackson is coming off a season in which he had 62 catches for 1,156 yards and 9 touchdowns for which he earned Pro Bowl honors in 2009. Maclin, for his part, was able to add 56 catches for 773 yards in his rookie year.

Once again, Shields wasn't showing any anxiety at the prospect of facing such dangerous and formidable receivers.

"I just think that they're the same [as me]," said Shields. "They run just like I run. They play football just like I play football. I went against a lot of good guys [in the past]."

Shields figures to have a short leash. He may have the nickel cornerback job to himself on Sunday, although if he doesn't play well, the Packers can always turn back to Brandon Underwood.

Underwood was limited during practice on Wednesday, practicing with a harness, and may or may not play against the Eagles. But even if he doesn't play on Sunday, he's a candidate to overtake Shields if the rookie fares poorly.

Both are subject to demotion if and when Harris returns to action following Week 6 of the season when he's eligible to play once again.