It's a weird trade because it's a trade about the post-Big 3 era, not about this year.
Jeff Green is a good player, probably better than Perk (hard to do a direct comparison), but certainly better than watching Perk walk this offseason.
But, Green is a starting-caliber player, and there aren't THAT many minutes to be had.  His abilities, this season, will be underutilized just because he won't be on the floor that much.
Perk, though...  downgrading from Perk to the Injury Twins or Troy Murphy or 'Sheed, that's a huge downgrade - huge.  Hurts us this year.