Well what exactly defines "a season like last year's Phoenix Suns"? Is it the record? Is it being in a race for 8th? Is it approaching the bottom quarter of the league in attendance?
I think the sentiment the OP was trying to evoke was one of over-achievement. Most prognosticators felt the Suns would be one of the worst teams in the league last season. They obviously were not. Phoenix made some major strides last season, and a lot of that can be attributed to the growth of their young talent. Phoenix did also get significant contributions from their experienced vets though too.
In order for Boston to replicate that, I think we'd need to see Sully and KO become more consistent outside shooters and for their overall games to grow. Boston would also need notable performances from Rondo, Green and Bradley (vets). Stylistically, it won't be the same, but the results could be similar. And if he C's can approach Phoenix's win total from last year they might even make the playoffs in the East. But mostly, I'm interested in player-growth.
Boston is at a point where they need their youth to improve and build up trade value (if nothing else). They don't need to tank for more unproven rookies. Another bad season would mean our assets continue to have marginal worth. That has to change for the next jump to be made.