Harrell, 24, had agreed to step in for Sean Brady at late notice to face Della Maddalena in Las Vegas this weekend.
But he was forced to pull out after his pre-fight MRI scan detected a rare brain disease that will require surgery.
Speaking after Harrell’s withdrawal, his manager Maurice Blanco said: “Josiah Harrell’s pre-fight brain MRI may have saved his life.
“He was diagnosed with moyamoya syndrome, a rare disease that results in blocked arteries at the base of the brain.
“He will undergo a brain procedure soon.”
Harrell had already weighed in for the fight before learning of his scan results.
The 7-0 fighter had been set to make his UFC debut this weekend.
His last fight was under a month ago, when he defeated Mike Roberts on June 16 in the LFA promotion.
This week was the first time in Harrell’s career that he had been required to undergo a mandatory brain scan ahead of a fight.
Harrell is not the first UFC fighter to have been diagnosed with moyayama disease.
The condition, which causes blocked arteries in the brain, was diagnosed in former UFC bantamweight Vince Murdock in 2019.
He was able to make a full recovery following surgery and has since resumed his career.
Blanco added that Harrell is “used to adversity” and that “he’ll be back” following his recovery.