I've send an email to Sigames in august if fans opinion have an impact on player's morale, and they reply to me in this way:
"The supporters can affect the player's morale, but it is usually not the main factor in deciding their morale".
So i think that they could influence their morale in two different situations:
1) the player is a favourite staff/icon/legend and they have a positive opinion about them (and you can check also in staff report at the voice "fan perception")
2) with the comments about the performances. I've seen in comments that they are positive if they are favourite staff/icon/legend. But also in the case that their opinion is good over 80% in the blu line, even if they aren't in the favourite staff and so on. And they're negative under a certain percentual (i've not defined so far), or when the trust of the supporters in match is near 0% and they are in rage about the result and the bad performances about a player (and player morale drop down).