I use academy players very frequently, and sign those that can replace my Auction players. I don't power train but i play them twice a day and train them twice a day to get as many cones for them I can.

Having them injured during training is normal, you jst have to manage your red packs. Like I said before, a player who gets injured very so often is an indication that he is your best player and the game just wants you to use up your red packs. Again, if it requires max pack to heal, then heal him. Else, you can afford to let him ride it out and lose a couple of days.