To guarantee that a player plays 48 minutes at a position, normally you just need to set him at starter, backup, and reserve. However, if you are winning by 20+ points in the fourth quarter, the coach will try to take every starter out of the game. To prevent this, you should avoid dressing any players who aren't being used as a starter or a backup.
For example, if you are forcing 48 minutes at one position, it is still ok to use 9 players. If you are forcing 48 minutes at two positions, you can use 8 players.