Among everyone's concerns for the upcoming season, this is one seems pretty consistent. It takes a team a while to come together. Sure, the '08 Celtics won a title in their first year together but they're thought of as the outlier. But are they?
When Lebron and Bosh went to Miami, they didn't really figure out how to best use everybody until the second season, this is true. But in their first year, they still won 70% of their games and cruised to the Finals, never losing more than 1 game in any Eastern conference playoff series.
When Lebron went back to Cleveland, he had to integrate with a whole new roster which included bringing in Kevin Love. They struggled (kind of) but still finished with the 2nd best record in the East and lost only 2 games in the Eastern conference playoffs before losing in the Finals with 2 of their 3 best players injured.
Shaq's first season in Miami ('05) didn't see a Finals appearance. But the team won 59 games, best in the East, and took the Piston to 7 in the ECF (missing Dwyane Wade for a game). From '04, they lost 4 of their top 6 guys in terms of mpg and in '05, 5 of their top 8 in mpg were new to the team.
Charles Barkley's only Finals appearance came as a member of the '93 Suns. A team sometimes talked about as one of the best to never win a title. It was Barkley's first year in Phoenix but it was also the first year for 5 of the top 7 guys in mpg during the playoffs for the Suns.
Going way back, the 1980 Lakers won in Magic's rookie year. They returned 3 big pieces from 1979 but the other 5 of the 8 players in their playoff rotation were not part of the team the season before (although we can quibble about Michael Cooper).
We can also look at smaller additions like Pau Gasol (the '08 Lakers) and Rasheed Wallace (the '04 Pistons) being added to teams at the trade deadline, having very little time to be integrated into their new situation and making it to the finals and winning a championship, respectively.
There isn't a lot of precedent for a team as good as the Celtics overhauling its roster this drastically but I tried to find teams that had some kind of success after big roster changes. It's hard to win a championship with an almost completely new roster but it's hard to win a championship anytime.
If your attitude if "Championship or bust" then I didn't find anything, historically, to make you feel better about the prospects of this Celtics team "gelling" fast enough. But if a successful '18 season, for you, would be a Finals appearance, I do think there are signs that teams can come together and win a lot of games (including playoff games) even when there's a big roster turnover.