From early posts following my questions and my own players experience i think i know how to avoid injuries:

1. Do not train them when below 80%
2. A player after finished an injury should return to the game slowly and not at once (Like- first game 30 minutes, second game- Half)
3. A player after finished an injury should return to training slowly and not at once (Like- some stretches, and not practice match)
4. Replace a player when Moral or Condition during game below 70%

Again, my own impression, nothing official