The way I see it, the Human Brain increases its efficiency (Assuming the 10% Brain Efficiency Myth is true) in such cases, to be able to do all the work that otherwise an entire Brian would have done.
I'm curious what would happen if we were to somehow gradually disable parts of the brain, ensuring all functions of the Human Body are still intact, and then suddenly bring all of it back.
To answer your question, I'd imagine that it likely depends on the timescale. People with TIAs have decrease in function that lasts minutes to hours, and on return of proper bloodflow, might take days to fully recover. You might see this as people who completely lose speech during the TIA, and then have word hunting for several days.
A brain that adapted to gradually diminished function, say as a result of microvascular changes, over the course of months to years would likely experience the same debilitation if there was a sudden permanent change.
It is not. It's one of those things that qualifies as "Not Even Wrong".