He'll probably get more minutes than you think given the other 2 bigs are TL and Al. I guess it's possible Grant Williams gets those minutes instead. That would be a good development.
Worrying about minutes in the regular season before the season starts is almost always a nothing burger. The top 8 will probably miss close to a season's worth of games combined.
The C's played 17380 minutes in 72 games last year. Using easy math, that means 10 players could have played 1738 minutes in a season or 24.1 minutes per game. Only 2 Celtics played more than 1738 minutes last year. 6 players averaged around 24 minutes per game or more, and that's not including Evan Fournier. Granted the C's had a crazy amount of missed games last year but it just goes to show you that breaking down 240 minutes amongst 15 healthy players is pointless.
Going by MPG, the following 8 players combined for 220.1 mpg. There are 240 mpg. Those 8 players are Tatum, Brown, Smart, Walker, TT, Theis, TL, PP. So by MPG, they played 91.7% of all minutes.
By Minutes played, the same 8 players combined for 11959 minutes, or 68.9% of all minutes played. Over the course of the season, that's a difference of 3978 minutes or 48.5 mpg.
I know when people say regular minutes they mean when the team is fully suited, will that player play. Teams are almost never fully suited though.