My hero was stabbed in his shoulder from the back. I don't want him to be kept overnight. I want him to be able to refuse to stay in the hospital. What would the ER docs do for him? The idea is he ducked and the man got him from behind.
There’s a lot of bone and muscle in the shoulder. The vessels to the arm are in the front beneath the collar bone, and therefore protected when the victim is stabbed from behind.
The shoulder blade protects the back of the shoulder and part of the rib cage. The muscles attached to the shoulder girdle (the “corner” of the shoulder where the outer ends of the shoulder blade and collar bone meet) can be damaged with a knife.
The ER docs will want an X-ray series of the wounded area and a chest X-ray. A blood count (to gauge blood loss) would be reasonable.
If the cut is deep enough and long enough, it'll require stitches. I'd suggest having the knife hit the meaty part of the shoulder (read: muscle) but not sever tendons. Tendon damage equals surgery and admission to the hospital.
A stab wound over the shoulder blade will protect your victim from the knife entering the chest cavity (and protect him from admission).
The wound is likely to require a shot of antibiotic. It’ll also require a tetanus shot if your hero hasn’t had one in the last five years. The wound will need to be kept clean and dry, and no showering (getting water on it) for a couple of days.
He’d be advised to come back for a wound check, and to have the stitches out in about ten days. Of course, he may decide to take them out himself. A common mistake people make when they do this is cut the knot off and leave the loop of the suture beneath the skin. Cut only one side of the suture below the knot—then the whole thing can be pulled out intact.