It's difficult that the NBA draft is before the free agency market. We would really like to know what the numbers are for possibly re-signing Rozier, Wanamaker, Morris and Theis (or renegotiate with Horford).
And a sign-and-trade with Rozier is just not possible then as the Celtics would likely want to make a trade for Davis at draft night. This to make sure that New Orleans can choose their obtained draft picks themselves. For this reason the Celtics need a big 'filler' for salary matching purposes and things get complicated. It makes a lot more sense to trade Horford than for instance Smart to keep a balanced roster.
Huge trade proposal:
To Boston: Anthony Davis, Jrue Holiday and E'twaun Moore
To New Orleans: Al Horford, Marcus Morris, Jaylen Brown, Terry Rozier, Robert Williams, Guerschon Yabusele, Jabari Bird, '19 Sacramento 1st, '19 Memphis 1st, '19 Clippers 1st and '21 Boston 1st.
After that resign Theis, new rotation looks like:
PG: Kyrie Irving, Marcus Smart
SG: Jrue Holiday, E'twaun Moore
SF: Jayson Tatum, Gordon Hayward
PF: Anthony Davis, Semi Ojeleye
C: Aron Baynes, Daniel Theis