Booker is a much better shooter & passer than Brown. The offensive fit between Tatum & Booker would be better too, as their roles would be more defined than with Tatum & Brown.
Yes, Greens box scores are not that impressive, but Green by definition is an elite role player. He makes everyone around him better. Why do you think Golden State have tolerated his nonsense? Why did they allow Thompson to walk but re-signed Green? It is because he is extremely important on both ends of the court.
Yes, Green is indeed a bully, but so was Garnett. Yes, Green is a rival, but so was Rodman before joining the Bulls. This Celtics team is dying for someone to motivate them into consistent elite play. Horford, Tatum, White & Holiday are more lead by example.
White
Booker
Tatum
Green
Porzingis
This starting five would be comprized of elite passing, something that Celtics basketball is all about.