Mushin no shin is the "mind of no mind." In self-defense, at the moment of physical attack, you want to be of "no mind."
Are reinforcements on the way? It's too late, who cares?
Is your attacker a gang member whose buddies will seek future revenge on you? Can't worry about that now.
Will the police blame you and arrest you for fighting back? Later, this might happen.
In defending yourself, you break the arm of your attacker. After leaving the emergency room, the attacker may call an ambulance-chasing lawyer to sue you. OK, these things happen.
You can plan. You can train. You can anticipate. But, at the moment of the physical clash, you'd better be of a single mind of no mind. Get the job done. Tomorrow is tomorrow. You put your opponent in pain. You distract him thus creating an escape opportunity. You escape. This is self-defense.
If you risk and win, you'll be happy. If you risk and lose, you'll be wiser.