Best case scenario would be the Celtics being NBA Champions and the only meaningful stat would be winning 16 playoff games. Everything else really doesn't matter.
I want you to imagine the current team doing that. I'm asking how you think that team you are imagining plays.
Okay so now for the serious answer but let me explain my first answer. In today's NBA environment with the Warrior dynasty seemingly over, any team with great talent, great coaching, talent that can develop and evolve into something even better and a rookie or two that could surprise everyone, has an outside chance this year. The NBA is wide open. So though the chances are extremely low, there is one best case scenario where the Celtics win it all. They might go 16-12 in the playoffs doing it but who cares?
Realistically what is their best case scenario? Kemba fitting in as well as Kyrie did his first 60 games here. Hayward returning to Utah Hayward. Tatum and Brown playing at an All-Star worthy level, even if not voted as such. Kanter being a double double machine and surprises with some very mediocre defense. Smart being Smart. Timelord becomes a rotational worthy player that gives the team a rim protector. A rookie plays at a possible top 5 Rookie of the Year candidate level. Team wins 50+ games in the regular season, is a top 3 in the East team and makes the ECF. That I think is this team's best case scenario.
I see this team more as a 45-49 win team that will struggle to be the defensive team Stevens expects out of his teams. I think the team will be very weak defending opposing team's quality bigs and that will cost a lot of wins. I think the lack of experienced NBA depth will show it's ugly head during the inevitable stretch of having a couple starters out at the same time for games at a time. I am not sure this team gets out of the 1st round of the playoffs but probably should get to play in the second round. I think you can pencil in Philly v Milwaukee for the ECFs barring injuries to Embiid or Giannis.
I like the championship scenario better.
C's put together a great regular season where the sum is greater than the parts, kinda like the '15 Hawks (0 superstars, 4 All-Stars, 60 wins) or '13 Nuggets (0 Superstars/All-Stars, 57 wins), or even last years Clippers (0 Superstars/All-Stars, 55 win pace after trading away Tobias). while Philly and Milwaukee have a couple of minor injuries and new fit issues that slow them down a little (plus everybody's gunning for them all year while the C's fly under the radar at first).
Kemba Walker continues in the recent Boston tradition of having their starting PG be All-NBA 2nd team (Isaiah in '17, Kyrie in '19, and maybe in '18 too if he wasn't injured). Gordan is back to Utah form, and Tatum and Brown took the step we were all hoping they'd take last year.
Boston finishes with 58 wins, Milwaukee and Philly 57 each.
Boston sweeps Brooklyn in the 1st round, beats the Pacers in 6 in round 2 while PHI/MIL beat up on each other in an epic 7 game series. Bucks pull it out, and the ECF goes down just like the Celtics/Sixers 2nd round match up in '18 where all the analysts/media are predicting Bucks in 4-5 but the C's take it in 5 as they finally figure out how to stop Middleton (while Giannis still does his thing, but rumor is he was playing on a pulled hamstring).
Out West teams with the mega stars (LAL/LAC/HOU/GS) don't actually make it to the Finals as POR/DEN/UTA actually displace them (no offense meant for Lillard or Jokic). C's beat whoever in 6.