It has been stated before that the AI champs uses to get a lot of injuries over time, because of quests or whatever. 

In my last game, at challenging, first time I battled enemy heroes, they already had 3-5 injuries... And one more after that battle

I think a way to heal injuries, but specially done by the AI could balance that. Maybe a spell attached to a certain technology (so it costs mana), or a scroll to be bought from the shop (so it costs Gildar...although it should cost mana too....). Each use only heals one injury...

Yeah, players may benefit from it too, but the most interested in it is the AI...

If you don't find acceptable to be used by players, then give each AI at least 5 scrolls to heal 5 of their many injuries...