Skills will decrease during an injury even if you practice them.
At 60% healthy, a freshman will expect to play if you made any promises. I try not to play my upperclassmen until they are 80% or better
A medical redshirt is only available if a player misses most of the season but i can't remember the percentage of games. However, you can't apply for it. WIS just gives it out of the goodness of their heart. (I've never seen it happen, though)