He had to fake it to make it. At least, that's what his coaches told him to do.
Among the litany of topics covered during Rosen's intense three days of rookie minicamp was how to handle his on-field demeanor -- especially in the huddle.
"Even if I don't know what's going on, at least act like I know what's going on to try and instill confidence," Rosen said. "You want to walk into that huddle and you want the guys to follow you."
Rosen doesn't need to try hard to create confidence. It comes natural for the 21-year-old, as he showed throughout last weekend.
The jitters of taking part in his first professional practice, coupled with the rust of not having played football since late December, were evident early Friday. But as the practice wore on, Rosen calmed down.
And that's when Cardinals coach Steve Wilks started seeing Rosen's confidence grow.
"To be able to make a bad throw here but also shake it off, hit the reset button and come back, that's what you want in your leader," Wilks said. "Most importantly, that's what you want in your quarterback."
Recovering from a setback is a part of Rosen's game that he developed as a young, standout tennis player in Southern California. Rosen said he learned as a child how to handle small defeats amid a broader victory: Even though he would ultimately win the match, he'd lose points along the way.
That taught him to quickly move past the mistakes, the negatives, the losses -- a trait that has come in quite handy throughout his football career.
It has been on display thus far during his young pro tenure. He looked "phenomenal" calling plays, said coach Steve Wilks, who added that Rosen did a "great job" taking control of the huddle, identifying coverages and getting the offense lined up.
As Rosen and his teammates learned a new offense, took the field for the first time in months and worked together for the first time, there were moments of uncertainty.
But Rosen's confidence -- or at least his displayed confidence in certain situations -- showed the Cardinals what's awaiting them once their quarterback of the future is given the ball.
"No one's perfect, but you just have to have a confident demeanor," Rosen said. "You have to walk out there, and you have to be decisive. I think indecisiveness is probably the biggest cause of turnovers. Am I going to this guy, that guy, whatever? What's my protection? If you're going to make a mistake, make it full-speed."
Wilks' confidence in Rosen is evident. Wilks said he won't let Rosen run wild, but he also said he won't try to control him.
"The kind of person that he is has allowed him to be the player that he is, so everything that we do from a personality standpoint, we get those guys to conform ... within the teamwork," Wilks said of Rosen. "So, he's going to be fine. I think he's good. I like his leadership. Like I told you from day one, he's wired a little different, and I like that."
After coaching Rosen for less than a week, Wilks doesn't think the QB is out to prove anything to anyone.
Rosen has learned to ignore the outside comments, thanks, in part, to being mentored by current and former players. They shared their experiences -- the good, the bad and the ugly -- with Rosen, telling him how they could've done certain things better.
"It's only hard if you make it hard," Rosen said. "If you're Googling your name every other day and trying to reach out to people and find out, like, you can make it hard for yourself, but as far as now, if you keep your phone limited to messages and calls, and you don't read too much, then the only thing that really matters are the guys in the building, and that's what I'm trying to keep it at right now."
That works for Wilks.
Rosen's confidence is just another facet of his personality the Cardinals knew they were getting when they traded up to draft him 10th overall.
"He's being his own person," Wilks said. "We have accepted him for the person that he is and the player that he is. So, I don't think we have a problem at all. We've had our conversations and our talks, and it's about football and staying focused and really staying concentrated on that."