Hold up. Little bit misinformed there my good man.
Firstly, Stamina and free throw training DOES NOT require playing minutes at any position. So this means if your main trainee is injured or its a short week, its a viable option. Otherwise, train single position trainees 48 minutes for optimal gains.
The minutes your starters, backups and reserves play is dependent on a number of factors, stamina, fouls and your coaching substitutions. I would suggest always keeping your substitutions to Strictly follow your depth chart and having your main trainee play Starter, Backup and Reserve. This will give him the best chance of getting that 48 minutes training and full training for that week.
I'm not sure if your familiar with potentials, ages and trainees yet, but if your new to the game, Ild always recommend training up 3 trainees whilst building your team and arena up over time.
Manager of the Bahamas National Team!