I think that is an overestimation of what Parker is going to play. Sure, if the Celtics need an extra offensive boost, he is in. If they need an extra defensive boost, Grant or Semi. If they just want solid play, I imagine we will see more Semi or Grant.
Honestly, 7 guys will get consistent minutes/roles:
Tatum
Brown
Walker
Smart
Rob Williams
Fournier
Thompson.
The rest will be matchup/who playing well based.
That is a decent 7 to go with. I think adding Pritchard as the 8th on the list is pretty non-controversial. He has been averaging about 16 min for March and April and I would expect that to continue. He is the back up PG. Sure, Smart could take those minutes as Fournier starts to see more time as a wing but I think we will continue to see Pritchard in that regular rotation tier of the bench, albeit less minutes than Fournier or Thompson (who both probably see more like 24 min).
After those 8 get their minutes, there are probably about 24 minutes left to be shared by Parker, GWill, Ojeleye, and Langford. I think it would be great if Parker played well enough to see 12-16 of those minutes. I think he is the perfect fit as a scoring swing for the remaining role, but it is likely going to be different combinations. All of these 9-12 slots are potentially useful players.
Then the end of the bench is Nesmith, Edwards, and likely Kornet. Probably garbage time only or in the event of multiple injury/illness. These are not horrible options for 13-15 on your team. Then our two-way guys, Waters has some usefulness, Tacko not so much, but hey, these are two-way players, you can't expect much.