If McMurphy were in a 'vegetative state' (think Sunny von Bulow), we would not have been sent back to the ward, but would have been in a hospital bed.
I think McMurphy was probably still recovering from whatever surgery had been performed (those scars had healed) and he was probably sedated as well. 'The Chief' knew that McMurphy would never be the same man and was therefore, for all intents and purposes, dead.

How would that scenario explain the scars though?

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I think the OP raises an excellent point.
I did some research on lobotomies, and from what I gathered, a lobotomy resulting in a vegetative state, as in McMurphy's case, was extremely rare.
But the book/movie is even more unrealistic than that, in that the book/movie imply that McMurphy's state was a deliberate punitive measure.
From the research that I did, conducting a lobotomy for the express purpose of producing a vegetative state of mind was never ever done.
In this sense, the book/movie is extremely untrue to life.
Scariest words in English: Were from the federal government and were here to help. R. Reagan

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Not a good representation of a transorbital labotomy. He would've been swollen and have had bruises for weeks.

It's perfectly believable that McMurphy could have ended up in the state he was in after his "operation". More than one lobotomy, be it due to a mistake or intention, was taken too far.

I don't think you know what a typo is.

I think more then a lobotomy was performed
I don't think you know the difference between THEN and THAN.

"the filmmakers took some artistic license to make it obvious to the audience that McMurphy was brain-damaged following surgery."
This ^^^
Love's turned to lust and blood's turned to dust in my heart.

Also, I believe a lobotomy just makes you more docile and does not turn you into the lifeless rag doll as Mac appeared to be at the end.
There was a lot of variation in outcome for lobotomies, depending on how much the rod was moved around inside the skull and how much hemorrhaging occurred.
In some cases, the person would be basically functional but somewhat off following the surgery. These were the small fraction of "success stories" which basically boiled down to minor rather than severe brain damage. In other cases, the brain damage would be more extreme (usually due to huge hematomas in the frontal cortex), and patients would be left in an almost vegetative state like McMurphy's.
As a case in point, consider John F. Kennedy's sister Rose. She was lobotomized, and required 24 hr. nursing care afterwards for the rest of her life. She was basically left with the mind of an infant. The Kennedy clan tried to keep the story under wraps, of course.
Much the same thing happened to playwright Tennessee Williams' sister.
Long sharp rods that resemble knitting needles are inserted above the eye sockets into the brain and are used to severe the frontal lobe from the rest of the brain. The scar on Mac's head would indicate that a much more radical procedure was done.
I'm not sure whether this is because lobotomies were sometimes performed by craniotomy rather than the postorbital rod, or whether the filmmakers took some artistic license to make it obvious to the audience that McMurphy was brain-damaged following surgery. A black eye wouldn't have made it as obvious, at least not to those unfamiliar with how lobotomies were performed.

^^ This

There are different kinds of lobotomy. The older procedure was one where the brain was drilled into. The kind you have described is called a Prefrontal Lobotomy. It was less messy and simpler to perform, but very dangerous, as a single slip could kill the patient.
James Oglethorpe: Philanthropist by day, Philanderer by night

If you don't have the mental capacity to comment on IMDB then why bother.

Whatever.