Fitness is not related to the chance of injury, in my view. The chance of injury more depends on shapes of your tactics. For example, your players are more vulnerable while playing with high tempo (like high pressing with attacking football). Also, Man-to-man marking of your opponent increase the probability of injury on your players. You can make relative assumptions based on your tactic choises and some attributes of opponent players(like aggression, marking, bravery etc.)

Heading and positioning are 2 of key attributes of your strikers. Besides corner and free-kicks, good heading and positioning increase the scoring probability of your strikers, while attacking down both flanks. 2D view of game does not show heading goals, but I've seen many statements after a goal of striker that the goal was a head goal.