If Favors is really available, Ainge needs to push every button available to get him. Idk if he's a guy you trade Smart for, I probably wouldn't, but you'd at least consider it. The kid is 23, He's already a really good player and he has the potential to be even more than that. Plus his contract is workable.
The Jazz have a long rumored interest in Bradley, who would be An absolute perfect fit on that team. They also seem to have a glut of bigs, while we have a glut of guards. Favors is the perfect kind of guy you pair with Olynyk. I would go for something like
BOS trades Avery Bradley, Jared Sullinger, Marcus Thornton, the 2016 Dallas 1st and the 2016 Miami 2nd to Utah for Derrick Favors and the 2016 (?) Golden State 1st
He would almost instantly make us the favorites to win the Atlantic and were still set up very well for the future. Just resigning Crowder and possibly Jerebko would be good moves, and anything else we could pull (Tobias Harris) of would be gravy. Still have all the Brooklyn picks, Smart, and all our own picks. We'd be left with..
Smart/Thomas/Rozier
Rozier/Young/Smart/Hunter
Turner/Crowder/Young
Favors/Olynyk/Jerebko/Mickey
Olynyk/Zeller/Favors
You label Favors as a 4 and KO as a 5 but in practice it's the opposite. Favors is a pretty good rim-protector, and I think combined with Mickey's skill-set we could have a more than adequate interior D.