Prince Amukamara puffed out his chest.

“Just posing,” safety Antrel Rolle recalled with a snicker.

This was on the practice field a few weeks ago, the Wednesday after the Giants’ Week 3 win against the Carolina Panthers.

Amukamara, last year’s first-round pick, had played nearly a complete game for the first time in his young NFL career. He had also finally satisfied a “rule” enforced by his elder teammates, that he was a rookie until one season and three games had passed.

So Jessie Armstead, the ex-Giants linebacker who is now a team consultant, kidded with Amukamara, pointing to where fellow defensive backs Rolle, Kenny Phillips and Corey Webster were kneeling down.

“What are you waiting for?” he said. “You’re part of them now. You’re a vet.”

Amukamara played along, prancing over to the middle of their circle.

“He plops his helmet down, and he just starts cheesing from ear to ear,” Rolle said. “We knew what that was about.”

It was about the maturation of Amukamara.

Against the Browns Sunday, the cornerback is expected to make his second NFL start, one relatively healthy piece of a battered secondary (Webster has a broken hand and a tight hamstring, Rolle is nursing a badly bruised knee and Phillips is out with a knee sprain).

For the past year and a half, Amukamara’s development has been a work in progress, slowed for reasons like the lockout, his late arrival to training camp and injuries. The video of his teammates dunking him in the cold tub during training camp, which went viral, raised questions about just how far he had come.

But fellow players and coaches say they see Amukamara turning an important corner, becoming the confident and aggressive player he was known to be at the University of Nebraska — and what they’ve tried to bring out of him at the NFL level. It’s good timing, since his role on the Giants has never been more important than it is now.

“That’s why he’s a first-round pick,” defensive coordinator Perry Fewell said. “He comes with an expectation that if he can help us and play for us and be solid at corner for us, then we’re a better defense.”

Taking the next step

On the first day of training camp at the University at Albany, secondary coach Peter Giunta asked his players to write down their goals for the season. Amukamara’s: Be a starter. Be the No. 1 cornerback on the team. Make the Pro Bowl.

They were the goals Giunta had hoped to see.

For the second straight year, though, Amukamara suffered a preseason injury. Sidelined by a high ankle sprain, he didn’t play until the third game at Carolina, and didn’t start until the fourth game at Philadelphia.

But it’s time for Amukamara to take the next step, so the Giants didn’t hesitate. Before last week’s Eagles game, they gave him a responsibility that Webster has held for years: Communicate the down and distance and the opponent’s personnel grouping to the rest of the defense before each snap.

Amukamara did just fine in practice, but in the heat of the division contest? “Too fast,” he admitted to Giunta, who signals the information from the sidelines. Webster picked up the slack, but Amukamara will try again this week.

“That will be the big thing, to get him over the hump where he can do it all the time now,” Giunta said. “Supposedly, he’s ready to take that next step. So it will be interesting to see when he can.”

Amukamara is quick to point out another mistake he made Sunday night: Not being quick enough to come down in run support on LeSean McCoy’s 34-yard run. But in his two games this season, he has earned kudos from his teammates and coaches for his play.

Giunta praised the way Amukamara used his hands at the line of scrimmage to disrupt Eagles receiver Jeremy Maclin, who had just one catch for seven yards. That has been a point of emphasis with Amukamara, as the coaches ready him for the quick and slippery receivers he’ll see at this level.

After Amukamara’s season debut against the Panthers, Rolle sought him out in the locker room and said, “You played an outstanding game.” He pointed to the way Amukamara aggressively broke on a dig route intended for Louis Murphy and swatted away the ball — following the advice of Rolle, who told him to watch for that very route and to jump it.

“He was playing with a lot of swagger, which is what we need from Prince,” Rolle said. “We all know he is an exceptional athlete, and he’s a smart player. Just having a little bit more ‘dog’ is going to take him a long way.”

Toughening up

Amukamara, in an endearing way, describes his personality as “smooth, chill and charismatic.” He figures this is why his teammates have teased and prodded him, seizing on quirks like how he keeps his mouthpiece in and buckled to his helmet even during idle times in practice — which Phillips said no one else has done “since we were like, 8.”

That’s only part of the story, though.

“He just wasn’t the stereotypical tough guy that we’ve come to know and expect around here,” linebacker Mathias Kiwanuka explained. “This is a game, but it’s not a child’s game. When you get out there on the field, you want to know that every individual is going to fight and kick and scratch and do whatever they have to do to win.

“And if you see somebody maybe not taking something as seriously in practice, or if you see somebody who allows himself to get punked on the field, you’ve got to tell him, ‘You know what, this is not acceptable.’ ”

The infamous cold-tub video — in which Amukamara was dumped into a vat of water by defensive end Jason Pierre-Paul, while another teammate yelled “Stand up for yourself!” — struck that chord. Giunta, whose brother is a fireman, chalked it up to the kind of healthy initiation rite that bands of individuals rely on to vet new members.

Amukamara had a tough and aggressive reputation at Nebraska, where he was the Big 12 Defensive Player of the Year. But when he came to the Giants, the offseason had been wiped out by the lockout, he missed time while his contract was finalized and then he broke a bone in his foot in his second practice of training camp.

Amukamara’s talent, Phillips said, is the reason his teammates haven’t given him a break. They aren’t afraid to yell at him on the practice field. Pierre-Paul says he’ll shout, “Let’s go! Tighten up!” The fellow first-rounder explains, “Prince has got to step up. This is his second year, man.”

More subtly, Amukamara has found mentors in the secondary: Rolle, who pulls him aside for one-on-one talks, on topics that sometimes include meeting expectations, and Webster, the Giants’ long-time No. 1 corner. Amukamara said he admired the Jets’ Darrelle Revis in college, but Webster has become his example, particularly because of the mental toughness that allows him to recover from bad plays.

They have encouraged Amukamara to show emotion after big plays. And when he was called for a bogus unnecessary roughness penalty against the Panthers, it somehow became a source of pride, prompting Rolle to declare, “No more cold tub!”

“However it got done, it got done,” Kiwanuka said. “And I wouldn’t test him if I was anybody else now.”

Living up to expectations

Amukamara’s agent, Todd France, called him the week before the Eagles game and told him to expect something in the mail. Soon, a wall-size print arrived, of Amukamara intercepting a Vince Young pass intended for DeSean Jackson last November.

The play came on one of Amukamara’s first NFL snaps, after his foot injury. The photograph reminded him of what he’s here to do: Make plays.

“It was very encouraging,” Amukamara said.

Amukamara needs no reminder of general manager Jerry Reese’s plain expectations for him: To play like a first-round draft pick. The pressure rose when Terrell Thomas sustained another season-ending anterior cruciate ligament injury in camp. It has risen again among a thinned secondary.

But Amukamara calls this a good pressure, explaining, “I enjoy it, I welcome it, I allow it.” The greatest pressure, he says, is from within: To prove to himself he can be the kind of player he has been in the past, on this level. The more he plays, the more he hopes that comes out.

“Reassuring myself,” Amukamara said, “that I do belong in this league.”

Amukamara’s maturation is only just beginning, but this is one important step: his teammates now see a player who not just belongs, but has the potential to elevate the level of their defense.

But so says Kiwanuka, “I won’t tell him that to his face.”

Jenny Vrentas: jvrentas@starledger.com; twitter.com/JennyVrentas