As it stands now I think it is Thomas, Smart, Hayward, Morris, and Horford, but I don't think Ainge is done. I think he will still try to be active in free agency and use the Room exception on a big man. Dedmon, Illyasova, Pachulia, Reed, McGee, and Mbah a Moute are all available and all might go for the room exception or less. Now not all of those players would start over Morris, but I think a couple of them might.
I also think Stevens will do a great deal of lineup tinkering and if Hayward at the 2 (with Crowder at the 3) or Crowder at the 4 (instead of Morris) works better, then Stevens wouldn't hesitate to do that.