Let's face it. The player that fills that 15th roster spot will not play a major role in the regular season, and certainly not in the playoffs. Danny Ainge has at least three routes he can take. He can fill a specific need, he can plan for the future or he can do nothing at all for the present.
One of the present needs is rebounding. Spencer Hawes is a big who can rebound and hit three-pointers. He could fill part of the gap left by Kelly Olynyk's departure. But Hawes is 29 years old, and has bounced around the league for his entire career.