I think Monroe and Sully would be a great combination. Both can crash the offensive boards, Sully can play the high post and feed Monroe, and Monroe can clean up the glass when Sully goes inside.
To me the problem would be on defense we have two post players with below average footspeed, meaning they would both need to play excellent position defense to be effective.
My problem with this deal is stylistically. Those two guys would be perfect for a slowed down offense, if they were surrounded by 3 and d guys. If you believe Rondo is our future then running that type of team would not jive.
Most importantly, the Pistons would probably want more than Green and another veteran and I think giving up a young asset for Monroe would be a great mistake.