That happen often mate. If you want to be 100% sure your trainee will get 48+ minutes (except injuries or 6 fouls in the game) you need to put just 5 players in the roster, every player should be on his position as sub and reserve too.
Otherwise there is a chance your trainee will be replaced or something like that so he get, 48, 47, 46... minutes.
Best regards!
This is where we hold them!