If the trade is something like Morris, Baynes, and Brown then I'd be fine with that. I'd give up Boston's 2018 1st as well and I might even throw in Rozier or Yabu if it was a deal breaker. I'd try to bring back some lesser players for depth as well so like Jordan Crawford, Cheik Diallo, etc.
So something like this I'd do
Morris, Baynes, Brown, Yabusele, Larkin, BOS 18 1st
for
Cousins, Crawford, Ajinca
which leaves Boston post-trade
PG - Irving, Rozier
SG - Smart, Crawford
SF - Hayward, Tatum, Nader
PF - Cousins, Ojeleye
C - Horford, Ajinca, Theis
Various other minor pieces could be traded, but I think that is a reasonable enough trade if the Cousins with Davis experience doesn't work. Of course Brown is a lot to give up for a potential half season rental of Cousins, but I think it is worth the risk as that team could realistically win the championship if it came together properly.