Looking for a word or phrase that refers to someone who is unaware of their state, as if they were dead or dreaming but not knowing it. Closest I've come is something like "walking dead"/"dead man walking" or "Dunning-Kruger" but ideally looking for a single word, less idiomatic. E.g., "His inability to execute his work made him into an impotent husk, a [person who is dead or dreaming but does not know it]." The intended use is more metaphorical than literal.
1: walking or having the habit of walking while asleep
2: resembling or having the characteristics of a sleepwalker : SLUGGISH
While meaning (1) is an actual, literal sleepwalker, meaning (2) is metaphorical. It can be applied to someone who goes through their daily routine without reflection, without thinking about what they are doing, someone who is in a dreamlike or unaware state but who can still move and act.
This is a common fictional trope, and TV Tropes calls it Dead All Along; it is found in many works of fiction, particularly horror films. The typical plot is that a character is unknowingly dead, wanders around with bad things happening, and in the end it is revealed that they were dead all along, and they leave the mortal realm, usually taken to some kind of afterlife (good or bad). I don't want to post spoilers so you can see their page for a more complete list, but well-known examples include The Sixth Sense (1999) and Carnival of Souls (1962).
The term also appears elsewhere, e.g. in movie website ScreenRant which has an article titled "15 Movies Where The Main Character Was Dead All Along".
A simple example of use would be "He wondered why lots of bad things were happening, but it turned out he was dead all along".
I think you can use the phrase he is as good as dead
As good as dead means he is unfit to do anything and he is almost equal to a dead person.
Here is the link