A few items led me to this idea. Raptors GM Masai Ujiri has prioritized becoming a better perimeter defensive team this summer, signing DeCarroll and Joseph. The Toronto Sun has also reported they're quite open to trading DeRozen due to his pending FA in 2016, where they don't want to give him max money. They also now need a starting PF.
So, as part of an overall agreement, Ainge and Ujiri do the following:
1) Raptors S&T Amir Johnson to Cs for David Lee (could be three way deal with GSW);
2) Raptors trade DeRozen and Terence Ross to Cs for Bradley, either KO or Sully, and one pick.
To me, this checks key boxes for both teams.
You're idea to immediately trade David Lee makes no sense. They just acquired the guy in exchange for a third-string player that suddenly become a legend since he was traded in Gerald Wallace.
I would have no interest in already dealing Lee, who will make an impact and improve our team.
I REALLY like your second idea and I know PhoSita had also mentioned something similar in another thread. I am especially really big on Derozan. He may not be a top 20 player per se, but I think he would be a HUGE upgrade for us over Bradley and his inconsistencies.
I am sure someone is going to whip out random stat that supposedly proves that Bradley is better than Derozan but I am going with what my eyes tell me.
If we could get him AND Ross, it would be a steal for the Celtics.
My only fear here is the Raptors willingness to trade within the division especially when a player of Derozan's caliber is involved.