TEMPE, Ariz. -- Josh Rosen has been a backup before, but it has been a while.

Before standing on the sideline for the Arizona Cardinals' season opener against Washington as Sam Bradford's backup on Sunday, the last time Rosen didn't play in a game when he was healthy was as a freshman in high school.

That was 2011. Rosen was 14. He was known more as a heralded tennis player than a future franchise quarterback. That year, he entered St. John Bosco High School, a prep powerhouse outside Los Angeles, as the most talented quarterback in the Braves' program -- but Jason Negro, Rosen's high school coach, said the young Rosen didn't realize it at the time. Instead, Rosen knew the Braves had a senior incumbent quarterback who'd start while Rosen sat behind him, starring on the freshman team.

Rosen claimed the starting job the next season and never looked back. He went on to start the next three seasons, winning a California Interscholastic Federation championship as a junior and capping his celebrated prep career with 8,473 yards and 90 touchdowns, before enrolling early at UCLA, where he won the starting job as a freshman.

But it was Rosen's freshman season at Bosco that Negro believes prepared Rosen to handle the highs and lows of his football career.

"For him to just be part of the program and absorb everything that was going on, he was totally fine with it," Negro said. "He understood that it was a process, and once he got his opportunity, he needed to take advantage of it."

Not much is different now.

As Rosen, seven years older, returns to Los Angeles for the first time as an NFL quarterback, he's sitting and waiting again. Instead of a celebrated homecoming for the No. 10 overall pick in this year's NFL draft, Rosen will make his first professional trip to his hometown as a backup.

As he did in high school, Rosen is absorbing everything around him and demonstrating the needed patience.

"I think he's patient, but I also think that he's very diligent in trying to learn what he needs to learn to be able to take that next step," Cardinals coach Steve Wilks said. "And to be able to have a guy like Sam [Bradford] as well as Mike [Glennon] to be able to learn from, I think is so important.

"Sometimes, these guys don't have that. I think Josh has expressed that in some point in time, that to be able to learn from these guys is so important. Sometimes, you get thrown into the fire and you got to deal with it and learn on the run."

And Rosen understands the situation he has been drafted into.

"I'm very fortunate that that is the standard I get to see coming into the NFL," Rosen said. "I'm not competing with someone who was a career backup and was maybe getting their shot here.

"[Bradford is] unbelievable, and I'm going to try and top that, whether now, later, whenever. I'm just very fortunate that that's the standard that I get to go see when I step foot in the NFL."

Those who have known Rosen the longest and the best have all said the same thing: He'll handle sitting as a rookie just fine. Even if it's relatively new territory, said Chargers center Scott Quessenberry, who was Rosen's center at UCLA and is a close friend.

Former UCLA coach Jim Mora, who recruited Rosen, has watched the quarterback's maturity develop. Mora thinks Rosen is at the point now where he understands his role is to support Bradford in any way, while waiting for his chance.

But that doesn't mean Rosen won't be anxious.

"I'm sure he'll be champing at the bit to get his opportunity to play, but I think at the same time, he'll do a great job of supporting Sam and the Cardinals team," Mora said. "That's kind of who Josh is. He really is a team guy. He's not selfish. He's not self-centered. He cares about his teammates, he cares about his team. He cares about winning. And I don't think it'll be an issue at all."

Then Mora stopped.

"Well, that's not true," Mora continued. "I'm sure that, internally, there might be a certain level of frustration that, 'Gosh, I wish I was playing,' or anxiety. But I know this: He did everything the right way to help his team have a chance to win."

Mora thinks Rosen will be more anxious than frustrated but will do a "great job" of hiding it. Coaching Rosen for almost three full seasons at UCLA showed Mora that Rosen was more than an outspoken, opinionated young man.

"He's always very realistic, but he's never negative," Mora said.

And Rosen's team-first mentality? That was often missed by observers, Mora said, adding he believes Rosen was misunderstood in college. People outside of the Bruins' program gravitated to Rosen's controversial quotes, Mora said. They didn't know that Rosen made sure to find an apartment close to campus with a big deck and plenty of room to entertain. They didn't know that Rosen took teammates who couldn't get home for the holidays to his house so they could celebrate. They didn't know that Rosen was "really invested" in his teammates, to the point where he'd talk with Mora about situations he felt the team could help. They didn't know that Rosen was voted a weekly captain -- typically unanimously.

"I don't think he was arrogant," Mora said. "I don't think that he was selfish. I don't think that he didn't work hard. I don't think that he didn't care about football. I think he cared about football. I think he cared desperately about his teammates. I think winning was important. I think he put the work in. I think that he is very, very, very, very tough. Yeah, he's had some injuries, but they're legitimate, legitimate injuries that happen in the game of football sometimes. I think he's one to play through pain.

"And I think that it was important to him that his teammates respected him and trusted him. He's a guy I've heard people say, 'He was a silver-spoon kid,' and his family background and he doesn't need football -- and he just never projected that at all. He was a guy who sometimes he would put himself out there publicly to bring awareness to some of his teammates that weren't as fortunate as him."

Former Bruins offensive coordinator and running backs coach Kennedy Polamalu, who's now the Minnesota Vikings' running backs coach, said Rosen's competitiveness is engrained in him. But that won't distract Rosen from his focus.

"He's going to find himself immersed in something -- the playbook, the concepts and everything else," Polamalu said. "He's a smart kid, and he understands where he's at right now."

Understanding his lot is one thing.

Negro thinks Rosen is on the edge of his seat, waiting.

"I think his patience is going to be tried here a little bit, because he is such a great competitor," he said.

However, those who played alongside Rosen don't think sitting will faze him much.

Former Bruins tackle Kolton Miller, a first-round pick of the Raiders, doesn't think sitting will be an issue for Rosen. And it won't set Rosen back, Miller said.

"Josh is a very mature kid," said Baltimore Ravens wide receiver Jordan Lasley, who played with Rosen at UCLA. "He's just resilient. He's calm. He's cool. He's collected all the time. You never know what's going through his head. He'll be fine.

"He's a patient guy by his own natural demeanor. One thing about Josh is he's very honest with himself. He'll know internally whether he's ready to be the starting guy or he needs to be a backup and he still has to learn. Josh is a perfectionist. He doesn't want to go out there and kind of poop the bed instead of going out there and be totally successful. He's not going to set himself up that way."