Unlike Ellis, who's undersized and mentally lazy, Martin has defensive potential. Martin's problem is that he's a bit short-armed and takes possessions off in terms of intensity and effort - partially due to being so absorbed with the offense. He can always struggle with bigger players, but that's too much of a rarity to matter. Martin would improve to slightly above average defender in the Celtics.
Not worried about the injuries either. If he was victim of a recurrent shoulder/knee/whatever problem, then yeah, but so far they've been a product of bad luck.
What I don't see is a way of getting Martin. Perkins could entice them, but how would we replace Perkins?