No matter what you do, there is always a small chance that your player will not get the minutes that you believe they should. A few suggestions:
1. ALWAYS play strictly follow depth chart, when dealing with rookies minutes that you are trying to train.
2. When looking at getting certain players 48 minutes in one game, play with less players (For instance if you are training one player at the PG position, only play with 9 players so that no one can replace him in garbage time, or if you are training a PG and SG then only dress 8 players).
In your game you played with 10 guys so he was replaced in garbage time because you were wining by so much, next time do everything the same but just make sure you only dress 9 players and strictly follow the depth chart.
Murray/Harris/MPJ/Grant/Jokic - 2020 NBA Champs