I am not sure you can really a player a bust when they are outside of the lottery.
Teams get value out of mid-late 1st round picks all the time.
If you miss on a pick in the 25-30 range (e.g. RJ Hunter, JaJuan Johnson, etc) then that's fair enough, I suppose. If you never find value in that range, you perhaps don't have as much of an excuse, but that's always a low percentage range to draft at.
15-20, or even 20-25 though, in that range there's always a good number of players available who end up contributing to teams.
So for example, the fact that James Young was never a useful NBA player isn't shocking nor is it a disaster for the Celtics, but it's also fair to point out that half a dozen players taken in the 1st round after James Young have earned playing time in the NBA and been at least somewhat useful.
The bottom line with Yabusele is that the greatest value he offered to the Celtics as a draft prospect was that he was willing to stay overseas for a year and not eat into the cap.
It would have been better if Ainge could have traded that pick for future value, but my assumption is that the offers weren't good.