Wonder no more.
Because if his sprinting out of the blue medical tent and back onto the field in the first quarter after taking a nasty blow to the head to throw a touchdown pass wasn't enough to rally his new guys, there was his gritting through his surgically repaired left foot getting rolled up on and not coming off the field.
Then there was the way he rebounded from a potentially crushing end-zone interception late in the third quarter to lead a go-ahead fourth-quarter drive, let alone his clutch 8-yard scramble on third-and-7 to clinch the 17-16 win over the Denver Broncos.
Yes, there were a lot of smiles and compliments in Las Vegas' locker room after its white-knuckle victory, which came in the first Raiders season opener started by a quarterback not named Derek Carr since 2013.
Especially from All-Pro running back Josh Jacobs, whose blocking enabled Garoppolo to scramble for the game-clincher.
"Man, he's a dog," Jacobs said. "Any quarterback that's willing to take a hit in a big moment to secure a win got my respect."
Garoppolo had a workmanlike day throwing the ball, completing 20 of 26 passes for 200 yards with two touchdowns and the interception for a 107.9 passer rating. He was not sacked, either.
"I mean, it's a terrible, stupid decision," Garoppolo said. "Just got to take the points. Throw it away when nothing's there. But I thought mental toughness kicked in and guys ... believed in me, so I appreciate them."
The feeling, obviously, was mutual, what with so many canine comparisons.
"He's a dog," said Raiders defensive end Maxx Crosby, echoing Jacobs' description. "We see that every day in practice. He put it on display today. It's exciting to see."
Another bone to chew on is how the Raiders were able to hold onto this game after faltering in so many similar situations last year. They lost nine games by seven points or fewer last season, dropping five of the 10 games in which they led by at least seven points.
"This type of game was the difference in our season last year, just finishing," said receiver Davante Adams, who had six catches for 66 yards. "They say you're trash when you don't ... we finish these and then they talk a lot different about you, regardless of what it took to get there, if it was ugly or not."
Again, a major difference? A new face at the most important position in team sports.
Garoppolo was 4-of-5 for 60 yards on the Raiders' go-ahead drive with a 6-yard touchdown pass to Jakobi Meyers with 6:34 remaining in the fourth quarter, his second touchdown pass to Meyers. He had three completions on the drive of at least 10 air yards, his most on any drive since Week 6 of last season, per ESPN Stats & Information.
And Garoppolo also found a warm place in Adams' heart by coming to him to essentially apologize for missing him on a potential big gainer downfield.
"That, to me, that's like you arguing with your girl and she says, 'I'm sorry,'" Adams laughed. ""All right, let's watch a movie now. We good. I don't need to act pissed off for the rest of the night and all of that.' So it makes it real easy to grow with somebody and continue to go and try to win when you got somebody like that.
"Jimmy's a gamer, man ... I mean, it's fun going out there and playing with him ... just his cool, his poise, I mean he threw some dots, he hit Jakobi over the middle with some guys in his face, showing that type of toughness and resolve after going and throwing a pick in a situation where we obviously need points. That's what you need from one of your leaders."
Throughout his career, Garoppolo has built a reputation of being unflappable while being in command of the huddle. And, of course, for his looks and Q Rating.
And cornerback Nate Hobbs saw it all coalesce Sunday when it came to the fans.
"All the women were like, 'Jimmy! Jimmy!'" Hobbs shrieked while shaking his head. "He didn't even respond to it."
And on the field?
"He was proving all the doubters wrong," Hobbs added. "Silencing them.”
While winning over teammates, and a season opener at a division rival.