There's a secret formula/high wire balancing act. 70% health at one injury spot is the magic # to putting a guy back in/fully used. 2 injuries I don't mess with = neck (must be 70+) to play because in the past a neck injury would explode in much, much worse if played and concussion. Concussion feels like a trap due to the low week amount and the unknown. I feel like messing with that could destroy a player, therefore, I wait for 100% full health on it.
Multiple injury spots on a player. (74/70 lets say - foot/ankle = sit) Our boy is on record as saying the sim engine will get you if you play a player with multiple injuries. I do but only based on quality of opposing GM, situation and the stakes.