My take on how it works... morale will always move towards 'Good'. So if a player has a normal game and you win, but there's nothing for them that particularly improves their morale they will continue moving towards 'Good'. If their morale before the game is 'Very Good' or 'Superb' their morale will likely decrease as it moves towards 'Good'. If there's something about the game that increases their morale (where you will get a message in the Assistant Manager tab) that will effectively stop the movement to 'Good', holding them at 'Very Good' or 'Superb' if that's where they started, or moving a player from 'Good' to 'Very Good'. If there's something about the game that decreases their morale that will speed up the movement to 'Good', or take a player down to 'Poor' or 'Very Poor'.
So morale is going to continue going down if you have high morale at the start of the game, and it will be impossible to maintain 'Superb' morale through performances alone - that will always require blue packs. At best, with a good run of strong performances it looks like you should be able to maintain most of your team at 'Very Good'.
