I think Danny added the player that would best help the Celtics at 16, so in a way, he did draft the best player available. His the thing, Yab probably has the highest ceiling of anyone chosen after him and can be stashed. The Celtics needed exactly that.
They needed a high ceiling guy to swing for the fences on given they have several players on the roster who are young and who's ceiling are those of role players. Yabusele has starter to All-Star ceiling potential if he continues to stay in great shape, learns the game more, and develops his skills. The C's needed to come out of this draft with two guys with that type of ceiling.
Unfortunately for Yabusele, has has only been playing the game for 4 years. His basketball knowledge probably isn't high enough right now(and probably won't be for two years) for him to warrant a roster spot on an NBA team. So he needs someplace to develop and overseas is the best option
Why is overseas the best option? Because as others have said the Celtics need the roster spot this year for a veteran if they are going to try to get past the first round of the playoffs and because then they don't have to play Yabusele. He's locked into a 2016 16th pick salary slot when he does get here. That will help the future cap situation.
Now could Ellenson, Brice or someone else possibly have helped this team sooner than two years from now? Possibly, but their upsides aren't as high as Yab and this team is filled with young players that really don't project as NBA starters and in Danny's book, and mine, neither did those guys