The first number in the % column is the percentage fatigue at which they are playing. If someone is overpacing their real life plate appearances by more than 10%, they begin to experience fatigue, and their performance begins to degrade. The number in parenthesis is what their fatigue level will be after the next game.
As far as injuries, they only happen for one game. Multi-game injuries are not a part of the sim. The real-life plate appearances for each player already factors in how many games a player missed, whether by injury or some other reason.