Generally, it is difficult to notice a player gaining morale after a match.

But if a player is getting great form (8+) you'd better not do anything to his morale bar.

From my experience, although morale boost cannot really fix a bad form player (as compared to green pack supplement and extra training significantly boost performance), boosting morale can destroy a player in perfectly good form if you are careless.

It is a thing you won't easily notice until you can keep a full team of 7+ rating in almost every match, which itself is a testament in managing skills.