I can see points both ways here, and I'd prefer for guys to be getting fewer minutes. As I see it, the NBA regular season is for:
1. making sure you comfortably get into the playoffs
2. building good habits such that you can "flip the switch" to engage those habits fully, come playoff time
3. developing players, both overall (young guys) and in the specific roles you want them to be played in (vets)
4. figuring out schemes and lineup combos that work in different situations
5. staying healthy while being "stretched out" enough to play big minutes in May
Having Tatum, Brown, and Al play fewer regular season minutes doesn't seem to conflict with any of those goals, particularly when the team looks on track to bank the wins for (1).
That said, minutes are one coaching decision that can be managed from the GM spot, to a degree. If Brad isn't happy, it's up to him to talk to Mazzulla, make him comfortable with job security, and get on the same page for the long haul of the season. I can't put this one on Mazz fully, one way or another.