Biyambo can actually play center & Jonas is a terrible defender so they won't lose much if they get a PF that can score instead of someone like Faried who's like a richman's Biyambo. I'm not sure Markieff works when they play Carroll at the 4 along w/ CoJo/DeRozan/Lowry since he isn't a rim protector.

They can acquire Horford by essentially swapping him for Jonas: http://espn.go.com/nba/tradeMachine?tradeId=znef6mj

The Hawks can pair Jonas w/ Millsap who is a very good defender & they also have Splitter who's a defensive minded center. Jonas gives them a post presence & a solid rebounder which is what they are lacking. He's also young enough (23) to be part of a rebuilding team if they want to go that route.