John didn´t see the attack coming until it was nearly too late.
Perhaps it was because he hadn´t slept for days while accompanying Sherlock during his latest case, or because he was on his way home from a double shift at the surgery and had his arms full of grocery bags. (Sherlock, the ignorant git, had disregarded his turn to do the shopping again.)
When John realized he had been followed, the man was already lunging at him. John turned and slammed the bags into the attacker, but his aim was slightly off. He was lucky the knife plunged into the groceries instead of his body. The momentum of the criminal threw him off balance, and John hit the pavement, bad shoulder first. He tried to ignore the stabbing pain. His immediate problem was the man on top of him who was reaching for the knife again.
But before John could react there was a flash of black coat and curls and the attacker was on the ground.
John sat up, breathing rapidly. The doctor part of John´s brain thought a moment about stopping Sherlock, but got ignored because the rest of his consciousness was occupied with the sight in front of him: Sherlock, mopping the floor with John´s attacker, growling: "Don´t you dare to hurt my blogger."
It was not the first time he had to clean up the mess his consulting detective had left behind. But it was the first time he´d gotten a phone call from a confused and absent-sounding John Watson. It was the first time he had to literally tear Sherlock away from the screaming suspect he was roughing up.
Lestrade had known Sherlock for years, but had never seen anything that came even close to provoking such a reaction. Obviously, the criminal had tried to attack John Watson.
Lestrade had asked Sherlock if he hadn´t overdone it a little bit, but Sherlock seemed perfectly content that the attacker had gotten an ambulance ride to the hospital and some injuries that would definitely never heal completely.
"A lot of criminals I antagonised think it easier to hurt a small, harmless-looking doctor instead of going for me directly. They have to learn that even trying to harm John has very, very serious consequences," Sherlock had explained earnestly.
Lestrade shook his head in silence while watching the pale hands Sherlock had beaten a man into a bloody pulp with roaming over John Watson gently, examining every bone, every bruise. The DI smiled when John tried to beware some dignity while Sherlock insisted to wrap him into an orange shock blanket.