I think the two key points that have been made are (1) there aren't going to be very many slots for FAs, and (2) any slot that may be open is for a bench guy at best.
To recap, the Celtics have 9 players signed for next season:
Brown, Tatum, Holiday, Porzingis, White, Horford, Pritchard, Springer, Walsh
Two with team options (very friendly options):
Queta, Hauser
And one player option:
Brissett
There are 3 players that become UFAs, Kornet, Tillman, Mykhailiuk.
I don't think Queta, Hauser, or Brissett are likely to leave. That means we have 3 opportunities to keep what we have (Door #1) or try to upgrade via FA (Door #2). And as others have said, we have to likely absorb a rookie first round pick.
Would you rather have Drummond over Kornet or Tillman? Kornet and Tillman are both UFA so they can do whatever they want. I am not expecting much to change. The change will probably be that Tillman and Springer end up with bigger roles.