Quoting SpankTheMonkey, reply 1
They are semi-permanent. You can buy (with the appropriate building) or find Iru's Elixer. This will heal your champion as well as remove a random injury they may have.
Damn, I am playing my first game (have been playing it for a while, though) and I have basically treated many of my heroes as cannod fodder because I believed that they would basically just respawn at no cost other then having to wait for them to respawn.
Do heroes obtain these injuries and penalties only as a result of being defeated in a battle or can it happen simply by losing a lot of health in battle?
Also, I have noticed that if a hero is defeated in a battle but the army it is in wins then the hero won't actually be removed for a time like usual but is instead simply very wounded, When that happens can it still obtain injury traits?
It's cool live and learn. Champions will obtain injuries if they are defeated in battle, it has nothing to do with amount of health lost. And as far as your other question if you still win the battle with your army if the hero is defeated, you won't have to wait the respawn time but he/she will still get an injury.
Best advise is to not let them die, even though it does happen. Also keep in mind that yeah injuries suck, but some are worse than others. The AI has the same thing too though so if you keep beating up on an enemy champion they will keep accumulating injuries and be less effective in battle.
Hope this help!!