They absolutely know how "good" he is, but how easily he reacts to managing the ups and downs of a full NFL game is different. You don't know about someone's fight or flight response until they are tested. Sometimes it crushes people, sometimes it elevates them.
Granted, he wouldn't be playing in the super bowl, but leading an NFL team as your audition to a possible career is still a big deal.
OTOH if some fans are saying that he shouldn't play because he isn't good enough to be an nfl qb, then why is he even on the roster after 2 years?