Danny biggest gamble? Yes, by far.
The KG and Ray Allen trades are complete opposites of this deal. In the prior transactions Danny traded away lesser talent for better talent. Especially of note, is that in both cases he acquired more accurate shooters. In the Fultz deal he trades away the consensus better player/shooter for one with poorer shooting skills, plus a (likely) lesser player to be named later. This is much more than a subtle difference.
The safer route would have been to keep Fultz. If he didn't pan out, then he could have fallen back on the excuse that he didn't live up to the hype. This way, if Fultz turns out to be a superstar, and the player he selects is not as good, then the blame will be on him and his staff. The good news is that this player should be pretty decent according to Danny and his staff. How much credit he’ll get for that is to be determined.
What makes this deal even tougher for Danny is that Fultz is going to Philadelphia. If the 76ers are healthy, then Fultz will have the advantage of playing with elite players. This should make him look even better than if he had landed in Boston. (a synergistic effect)
For example, he might be able to cheat more on defense with Embiid and Okafor beneath the basket. And, if he does have defensive deficiencies they won't be as noticeable as if he were playing here.
Add to this, Simmons looks to be a transcendent player with exceptional passing and ball handling skills. He's surely going to be able to drive to the hoop and kick it out to an open Fultz.
Saric is another star player that will mesh well with Fultz's skill set.
Markelle has the potential to be working in a well oiled system. Thus, if Philly is healthy he's going to look great. Danny has no control over that. Yet, he could end up being blamed for making it possible. This might not be fair in that most people will be comparing two players in different situations.
Boston will likely continue struggling with rebounding and shooting. Bringing in another mediocre shooter won't improve the situation. For some reason (in most cases) great offense seems to trump poor defense. Look at Isaiah, he kills the team defensively, yet he scores. So, its overlooked. When you read most of the articles on CB, they seldom mention a player's poor defense. Poor shooting is shouted out all the time. Thus, even if the player he selects is better in almost every other category....shooting is what the fans might note the most.