Unfortunately the Celtics started the rebuild at a particularly bad time due to the cap rising the next couple years. Teams won't have problems affording all their players and besides, they already had an advantage of being able to offer the most money. The teams that are good will just have the ability to get better or preserve what they already had and the bad teams have to build through the draft and/or sign the leftovers (sorry, Amir Johnson).
Still, 2 years is not a long time to rebuild so we have to be a little patient. It would just be a lot more promising if we had gotten at least one player with multiple all-star potential in those two years because you really need stars to lure other stars. The attraction can't only be that we have a strong organization, because lots of teams, especially the good ones, can offer the same plus other perks like nice weather and no state tax.
The cap rising and the 2016 draft supposedly not being strong doesn't bode well for the next calendar year. Since the team has the disadvantage of being a bad/mediocre team in a bad conference, we can't really suck enough for a great pick. The best chance now seems to be continuing to play well in the crappy East and hoping that somebody takes notice and somehow takes a chance on the Celtics. Since the FA class isn't so great in 2016, it could be 2017 when we have a shot.
The other possibility is that we luck into some great pick from Brooklyn which we either use on Simmons or some other good prospect, or we use our 2016 picks (our own, brooklyn, and possibly Dallas) to make some deal. I just don't think there will be a young enough star becoming available, however, and an old one won't really be the best fit with our young team. That leads us back to our original problem of there just not being lots of available stars unless they're already nearing their end peak years (Melo for instance).