Unexpected means it was something you did not "expect" to happen such as a car crashing into you which means this is wrong. Unfair is any injury. Unnecessary means it was never needed and could have been prevented/not done therefore unnecessary is the best term.
We can define as “accident” an unintentional and preventable event that causes physical and emotional injuries, occurring in the home or social environment. Prevention is defined as actions taken to prevent the occurrence of the injury and its strategies are aimed at reducing exposure to risk factors.
If an injury is unnecessary it means that i could not happen if the actions to prevent that were taken, which means, that this injury is preventable.
