Projected BPMs for the Celtics guys based on summer league stats only. I've included everyone from the Celtics who got over 100 minutes, but this model is really fitted to project rookie year BPMs, and doesn't discount guys for repeating the league, so it probably overrates Nader, Jackson, and Jaylen:
Zizic is one of five rookies with a positive BPM projected. Lonzo Ball leads all rookies in projected BPM by far (and has the best projection since 2008 overall). That's a great number on Tatum too. If he's close to an average player as a rookie, then that's well on his way to being a star.
This is mostly a toy - overall out of sample R^2 with this model is about 0.2, which is better than random, but is not especially strong. Most of the top performers in past years did go on to have strong rookie seasons however.