McIlroy had suggested after a disappointing 22nd-place finish in the PGA Championship that he might not play for the rest of the year.
He said the effects of a broken rib suffered early in the season still caused difficulties in his practice and had led to back spasms.
But the 28-year-old decided to seek a defence of his FedEx Cup crown after meeting with fitness adviser Steve McGregor in Belfast.
McIlroy won the second playoff event and the Tour Championship to emerge victorious in the FedEx Cup last year.
Like last year, he has not won a title prior to the playoff series.
This season's series opens with the Northern Trust in New York next week, followed by the Dell Technologies Championship in Boston a week later.
After a week off, the series starts again with the BMW Championship before the top 30 in the FedEx Cup standings advance to the Tour Championship.