Drafting is a crapshoot at best, especially with how young the NBA rookies are. No one even knows if Wiggins will be good much less James Young. The thing with the Celtics for the next few years is that they have so many picks, they can take chances and hope to get lucky along the way. I suspect they will make one "safe" pick and one "upside" pick each year that they have 2 or more picks.
Most of these upside picks will "probably" end up being no better than Gerald Green but if they do get lucky even once, that may be all they need. The "safe" picks are not sure things either of course so please don't over analyze my theory.
In terms of Young, I think he is a great upside pick to take a chance on. Better than Exum?; probably not but Exum could be the next Rubio or the next Tony Parker, who knows. My point is this comparison may be more about how bad Exum ends up vs. how good Young ends up. If both are bad and Young is a little less bad, it doesn't help the Celtics.