I'm pretty much always in favor of teams investing in their young players, but in the Celtics' case I really feel like Kadeem Allen's roster spot could be better filled by a vet who could actually come in and play 15 mpg when guys are hurt. Part of it, I'm sure, is that I'm just not convinced Allen's very good, whereas Jabari Bird is at least playing well in the GLeague.
Barbosa, Jordan Hill, Anthony Morrow, Jordan Crawford, Toney Douglas, maybe even Cavs rejects Deron or Derrick Williams -- one of these guys could help. On a team that already has Tatum, Theis, Ojeleye, Nader, Yabusele, and Bird as rookies, I think the experience and decision-making of one of these guys would be valuable when one or more of the Celtics' vets is out.
Edit: I just realized Allen is actually on a two-way deal, so this idea wouldn't work. They'd have to waive Jabari Bird or Guerschon Yabusele to add a vet.