I am going to try and present this another way. First eliminate the obvious or easy:
Smart and Pritchard will share 48 PG minutes (Probably Smart 32, Pritchard 16)
Horford and RWilliams will share 48 Center minutes (Probably 24/24)
That leaves 144 minutes for the wings, swings, power forwards, whatever you want to call them. From this 144 minutes, the following is pretty obvious:
Tatum 36, Brown 36, and Richardson 24 minutes. That is 96 minutes leaving just 48 minutes. These are the 48 minutes that are in question. These 48 minutes will be shared by the following (in no particular order)
That is on average 12 each. I suspect it will vary from game to game. Whether Richardson starts, Parker, Nesmith, doesn't matter, the minutes are going to work out the same. Richardson will play starter minutes (24 or more), Parker likely won't. I actually think match ups will force Parker and GWilliams to play more like 16 min per game, leaving less for Nesmith and Langford. There will be injuries also that will vary the distribution.
But as far as positional depth, it is how I presented it in my earlier post. Jabari Parker is a PF. Jason Tatum is a SF. Nesmith is a SF, Langford a SG. As sad as this is, Jabari Parker is the best player on the Celtics that plays the position of PF. Richardson by position is the second best SG behind Brown. So on a depth chart that is sorted by position, he goes in as the second SG. The coach can decide to play both Brown and Richardson at the same time but that does not change what their natural position is. You are just choosing to play with two SGs.
The bottom line is that Parker and GWilliams, two PFs, will play likely 24-32 minutes combined and they are not very good. If we had a better PF to play those minutes, the team would be better off for it.