I mean Rozier is averaging 2.6 assists a game as a point guard. Brown is at 1.5, and Tatum is at 1.7 despite playing 30+ minutes and holding the ball more than Rozier and Brown. None of those numbers are great, but at the same time I'm not really hugely concerned by them either. A) When and how to pass is often the last skill the develops for young players, especially young guys who up till this point in their career have never operated a lot with the ball in their hands as a play maker, all three of those guys got their first taste of I last year in a much more iso heavy setting (NBA playoffs). B) There's stuff to figure out role wise that has nothing to do with selfish or unselfish play. Those three young guys took this team to game seven of the ECF, maybe it is on them to readjust this year but its gonna take some time to figure out. Its further complicated by the fact that Brown is not hitting shots, and Hayward has himself been pretty bad this year which makes it harder to figure out the roles. C) Its not like Marcus Morris is exactly a great or wlling passer either, he has 1.5 assits a game. Bottom line is their best passers are great shooter (Hayward, Smart) and their best shooter aren't great play makers (Tatum, Morris) and that makes it hard to run an offense.
The TLDR version is this. I don't think its selfish play that is hurting the Celtics. Its a combination of the young guys not yet having the proper skills. With Brown its the fact he doesn't make the right play off the drive. With Rozier its he can't get the ball to the roll guy (or thro a lob) With Tatum its a lack of ability or willingness to get to the rim and too many isos. Combine that with confusion about roles. Combine both of those with key guys struggling and you get to where we are now.