If you have open roster spots, you might as well get this year's version of Tony Gaffney and Oliver Lafayette.
Speaking of which, Oliver Lafayette was in the D-League and doing pretty well. Then he signed by a Turkish team. But then that deal was rescinded because medical tests found a heart problem, although Lafayette may dispute that. So he should be available. Lafayette, at least, has some familiarity with the Celtics system.
For similar reasoning, Mario West is probably Ainge's Plan Z for a wing.