The way I see it, Bass is primarily a perimeter shooter who can take it to the rim on occasion. Green (if healthy... knock on wood) is arguably better at both of those facets of the game. The difference maker is defense and rebounding, both of which I think Bass has a slight edge in.
I've always wanted Bass as the first big man off the bench, and not a starter, but it's Doc's team and we'll see what he does with it.
Another question this raises... who plays backup time at the 3 if Green is playing most of his minutes at the 4? A few minutes to Lee, Green, and Bass, each?