I think we shouldn't talk about draft prospects as current players' substitutes. Rookies need to figure it out, so if Danny Ainge drafts any player who isn't a center at the NBA level it's because he's the BPA, not because he's the next Green, Rondo, Bradley or Sully/Olynyk.
If we had lottery luck, I'd love to get Embiid, Wiggins, Parker or Exum, but that's unlikely. With our pick, if everything goes as expected and we get a mid lottery pick, I'd love to take James Young, who has star potential to go with length and versatility as he could play SG/SF.
With Atlanta's pick I'd rather take a player with potential. Saric may be gone, but that's exactly the type of player you would like, a guy who can play the 3 or the 4 depending on matchups, who can be a good shooter and has handles, athleticism... or a guy like Cauley-Stein if he's still available.
Kyle Anderson would be great in the 2nd round, his body/athleticism will scare teams. His draft stock may rise, though, as his basketball skills seem to be great. Kyle, McAdoo and Isiah Austin are kids who could be 6th-7th players on good teams, and you need those, too.
Outside of Embiid, Wiggins, Parker and maybe Smart, I don't think any rookie would start on the Celtics from the get go. Maybe at the end of the season if they show they are capable to handle things. So it's a bit premature to relate who is available in the Draft with our current players future.