GREEN BAY, Wis. -- One rookie stepped onto the field with the Green Bay Packers' starters in his first day on the job.

Not first-rounder Kenny Clark, who couldn’t attend organized team activities because he was still in school at UCLA.

Not second-rounder Jason Spriggs, whom general manager Ted Thompson traded up to take.

Try fourth-round pick Blake Martinez, who has played inside linebacker with the No. 1 defense throughout OTAs and likely will remain there during this week’s mandatory minicamp.

With each passing day, Martinez looks less and less like a placeholder for Sam Barrington and more and more like a legitimate candidate to be an opening-day starter as a rookie -- something not even Clay Matthews did.

“It’s very difficult for a rookie to come in here and expect to start from Day 1,” said Matthews, who played only in specified sub packages early in his rookie year of 2009. “He’s coming along. He’s doing his best. Obviously, there’s going to be some learning along the way. But he’s proven that for the most part he’s making the right calls and signals. I think it’s a good thing for him.”

In the valuable game of offseason practice reps, Martinez has been perhaps the biggest winner in Green Bay.

“He looks very comfortable,” Packers coach Mike McCarthy said. “I think he’s done a really nice job transitioning from the base defense to the sub defense, his command, the echoing of the calls. He’s very bright. Quick. And he definitely is a very instinctive player. He’s off to a very good start.”

Don’t expect Barrington to give up the job without a fight but to this point, the fourth-year pro has done more off the field than he has on it. He played just 15 snaps last season before a foot injury in the season opener ended his year.

But a year’s worth of catching up awaits him upon his return.

Meanwhile, Martinez’s athleticism and ability to quickly absorb the defense might make it tough for defensive coordinator Dom Capers to yank him even when Barrington is cleared.

“What I do like about Blake is, No. 1, he’s very sharp -- which you expect a Stanford guy to be sharp,” Capers said. “And I think he’s very conscientious. He’s not afraid to ask questions. I think he’s going to be an attention-to-detail guy. He’s going to be a guy that’s going to be able to study the opponent and be able to play the percentages and probably gain a little bit because of that. The guys that I’ve been around that are smart and instinctive, a lot of times they pick up a step or two in anticipation.”

The real test for Martinez will come when the pads go on in training camp and reps are live in preseason games but at this point, he’s a step ahead of most rookies, who run with the second- or third-stringers in offseason practices.

Martinez not only has worked alongside Jake Ryan in the base and nickel formations but also as the lone inside linebacker in the dime package. With that position often comes the additional task of making the defensive calls.

“It’s kind of one of those things where they always talk about when you step onto this field, it’s going to be that change of speed from college to the NFL,” Martinez said. “I think it’s one of those things where I’m getting that speed instantly. The main thing they said is once you get that playbook down, you’re going to be able just see everything that much slower. I think being out there with those guys, having that communication, is starting to make things understandable out there instead of just being around all the guys who are still learning. So you get those little things that you wouldn’t get to hear when you’re with those other groups."