How critical is a tiny central disc protrusion?

Depends on symptoms: It depends on your symptoms. I would not expect a tiny central disc protrusion to cause symptoms. The specific symptoms you are looking for would be radiating leg pains or arm pains or numbness and tingling down the arms or legs.

Not at all: Very common and almost never of any concern. These actually tend to spontaneously resolve and you may not see it on an MRI a year from now. Structural changes of the disc such as bulges and protrusions are part of aging. Not unlike getting wrinkles or grey hair.

Usually not: Most tiny protrusions are part of normal anatomic findings which may be reported by a radiologist. The only concern would be in conditions where the spinal canal is narrow, such as with congenital spinal stenosis.

Usually not very: Small disc protrusions are usually not symptomatic because they are too small to cause mass effect on (or pinch) the spinal cord or nerve root. However, annular tears associated with or without a disc protrusion can be painful (although usually not critical).