ironically enough, the games are pre-determined when you are watching them! lol
you may not know, but the game engine for your game is actually run 15 minutes before the game is 'shown live'. This is why you can't change tactics in the 15 mins leading up to the game.
There also used to be a bug (not sure if it still exists) where if you are playing a cup game, the $50k will appear in your bank account, before the actual 'live game' is finished, thus meaning you have won your cup game.
So yer, the fact that the game engine is run in advance of you watching the game, this is also a pretty good reason for not having lve substitutions.