At night during the game update, or whatever, morale drops on at least a few players. This is random as far as I can tell.

But after a game morale drops if the player's rating is lower than one from his previous match. So if your players gets 10.0 rating and in the next match gets 8.2 his morale will most likely drop to very good or good. It will also go up if he gets a better rating, but the rise appears to be much smaller.

At least this is how it appears to me.

[tinfoilhat]I believe the morale does matter. It might be coincidence or confirmation bias but it FEELS like every time my team's overall morale is less than it was in the previous league/cl/cup match I struggle to get a result. I feel the same thing happens if I lower the win bonus. Who knows tho. That said, having morale maxed out on everyone doesn't seem to be ideal for good results.[/tinfoilhat]