Just because Justin Herbert is introverted, don't assume that he lacks confidence. Everything I've heard is that he is an extremely confident guy. Maybe having a really big arm is part of it.
For me it's not having the confidence that I think he lacks, its the ability to calm down a jittery huddle in the 4th quarter, or in the playoffs. That effusive belief that makes his teammates believe in themselves. It's hard to quantify or capture that ... but some guys just have it.
For instance, Joe Montana was a very good QB, and he was great at it. In a key early series in the 49'ers first Super Bowl appearance, the offense was overwhelmed by the moment. In the huddle, rather than call the crucial play, Montana said something to the effect (paraphrasing):
'Hey, did you guys see the rack on that girl in the first row!?!?!!? Wow, that's good stuff .... (pause) ... I'll call it at the line of scrimmage, on 2, BREAK.'
His teammates couldn't believe it, this was SERIOUS stuff, this was the SUPER BOWL, this was a CRUCIAL 3rd DOWN, and Joe was as cool as a cucumber ... he wasn't nervous at all ... he quietly and confidently knew they would convert the 3rd down and win the game. This calmed his offensive teammates down ... and they played well, and they won. That's just one moment, but you can see how it removed the nervousness from the entire huddle. That's a ****y/arrogant move. That made the team believe in him, and themselves.
Not every QB has that kind of leadership. For instance, Marino was the guy who displayed firey intensity, and on 4th and short, threw a bomb for a TD. After that, everyone on his team believed in him. Well, honestly, he had done stuff like that so many times before they already believed in him, but you get my drift. Can you see that sort of behavior out of Herbert? Honestly, I don't know, and that's why I'm concerned. I am confident that Fromm, Tua, Burrow, Love, and Eason can all command a huddle. I'm just not as convinced with Herbert ... he seems like a very nice guy, and that is not always respected in a huddle. Alpha dominance is what they want to see.