I think it's a bit of an overreaction to label a 19/20 year old timid after 1 season & a few exhibition games over the summer. There are very few people who wouldn't be somewhat hesitant when transitioning from competing against teenagers to professionals, not to mention doing so while simultaneously adjusting to a new country/culture. The fact that he was not only able to stick with the team for the entire year, but actually cracked the starting lineup (of a good team nonetheless) as a teenager is a feat in it of itself. I would be concerned if he hadn't displayed the necessary physical tools to stick in the league, but last year showed that he is more than ready physically (which is amazing considering his age), & the year before that (when he was playing against competition of the same age) he showed that aggression/attacking the rim is a major part of his game/playing style.
If anything, last year was extremely encouraging when you consider the fact that he was/is being asked to compete against grown men as a not yet fully developed 19/20 year old. I could understand being concerned had he not displayed a strong work ethic, but by all accounts, Exum is a very hard worker & a person of high character. It may take a couple more years for him to put it all together, but injuries are the only potential factor I see prohibiting him from becoming a pass 1st starting caliber PG with great to elite defense, court-vision, & (hopefully) slashing ability to go along with average to above average outside shooting (which would likely be a top 5 PG in the league). Once he catches up physically to the rest of the league, I expect to see that aggression return (if not increase) & (again) become an important aspect of his game.