Injuries are caused by many things. Tired players are most likely to get injured, though that is not always the reason, it plays a factor in most cases. As does how you play them, hard/attacking/tackling (YOURS OR THEIRS!) can cause injuries. Arrows can make players tired and more prone to injury. Too much training, and poor facilities can also play a part.

Ratings are harder to see the whys of.. mostly you can try playing them in different positions, with or without arrows, sometimes even out of position, and see if his rating get better. I find that it takes a few games to get more out of a player.