It seems that morale plays a higher role in FM2008 than in previous versions,
IMO.
If your star striker is on low morale, it's worth building him up again. Say you'll give him a rest for a couple of weeks, play him in easy matches or the reserves, be sure to praise him and have faith in his abilities during team talks, and so on.
If playing a less skilled player with high morale, e.g. a youth player, try to ease the pressure during team talks, be less demanding, so that you don't destroy his morale as well.