Is there a general consensus on what % a players health should be at before he returns to the lineup following an injury?  Just how big of a hit is it for a player to play injured, or is it mostly just a risk of them getting hurt worse?

At 70% health I'll use the player. At 80% I'll start him. I've played a couple of seasons and have never heard of an injured player getting injured (again). Increased chance of injury is supposed to happen and I'm sure it's happened but it is very very rare. The effect of injury is, for example, a 90% guy is 90% effective. Stamina, rebounding, everything. Actually, to be honest, if one of my players is injured I will check to see how bad and then I do nothing at all. If a guy is injured enough he will not be able to play and only then do I make line-up changes.

I play them at 80%.   I take 80% of his ratings and compare it to his backup. If the injured player is still better, I start him. Otherwise, he's the backup

depends on how much you need him - I start thinking about it at 75% or 80% - but you have to consider who else is available,

often, I bring them back initially lower on the depth chart - say if he was a starter I might have him come off the bench for a game since I figure the other guy may still be better but I want him there for depth and perhaps for improvement

also remember that if there are promises involved or if the player has recently complained about minutes they will expect to play once they hit 60%.