Well I was all set to say the following... Jeff Green is our 4th best player, but he's a SF. He's not a PF. And unfortunately our 2nd best player, Paul Pierce, plays SF too. Meanwhile we have a 3rd stringer starting at PF. So in theory, yes it makes sense to move Jeff Green for Josh Smith... it would give you a lineup of PG - Bradley, SG - Lee, SF - Pierce, PF - Josh SMith, C - KG
Not to mention, Josh Smith is just a far better player than Jeff Green.
But I just realized that Josh Smith is actually smaller than Jeff Green. That was surprising to me. Smith is 6'9 225 and Jeff Green is 6'9 235. You'd still have to trade Green for Smith, because Smith is a superior player... but it doesn't really help the theory that Jeff Green can't play PF.