First of all, this would never happen.
Boston would do well to bring back a regular all star in return for the pieces they have available. Kevin Durant is generally perceived as a top 2 player in the league. If you want to get him in a trade, you better be giving up a current or future superstar in return - think Anthony Davis, Paul George, Carmello Anthony, etc. Boston has no such talent. No team is giving up Kevin Durant for a tubby PF with a back problem, and some spare parts.
Secondly, as much as I couldn't realistically say no to bringing Durant in...I really dislike him as a player. He's just way too soft, way to weak, way too friendly, way too childish. He doesn't have the killer "take a bullet for this team" attitude that you want from a guy who's going to be the centre of your franchise. Just too soft, which is why I don't believe he will probably ever win anything. Unless he teams up with his own big 3.
In fact, I'd actually rather have Carmello than Durant. Yes he's a prima donna, but at least the guy plays with some toughness and some grit.
Plus Durant defensively is terrible. Carmello isn't a great defender, but at least has the strength to muscle guys on defence. Plus he can dominate opposing players in the post, a talent Durant with his 'Tayshaun Prince' frame lacks.