McIlroy, 28, says a recent medical had revealed the issue, which he has described as "not that big of a deal".
"The rib's fine, no problems whatsoever - I had an MRI scan on my thoracic spine and all was OK".
"For now, I just need to stay on top of it and have to stay fit".
However, it turns out that McIlroy's health issues weren't confined to his ribs.
In an interview with the Daily Telegraph, the world number 11 said the abnormality may be linked to a viral infection he contracted 18 months ago. "Hey, I was planning on doing that anyway". "I just crave to be in that position again". The Northern Irishman will begin his 2018 season at the Abu Dhabi HSBC Championship, starting January 15, where he is a four-time runner-up.
McIlroy fell from No. 2 in the Official World Golf Rankings at the beginning of 2017 to No. 11 at the start of 2018 after not winning previous year.
McIlroy will tee-up next Thursday in the UAE capital without a victory in 2017 and not having tasted success since capturing the PGA Tour's season-ending 2016 Tour Championship and the week prior the Hazeltine Ryder Cup.
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You can read McIlroy's full interview with the Telegraph here. McIlroy skipped four scheduled starts and didn't return until March.
"I shouldn't have played the U.S. Open, " said McIlroy, who missed the cut.
Rory McIlroy isn't letting a newly disclosed heart condition derail his 2018 goals: winning more majors, and reclaiming the world no. 1 ranking.
"Mentally more than physically probably".
Speaking with the Telegraph's James Corrigan, the four-time major victor revealed that doctors discovered an irregularity with his heart.
McIlroy will face stiff competition at the Abu Dhabi HSBC Championship from a field which includes world No 1 Dustin Johnson, Race to Dubai victor - and defending Abu Dhabi champion - Tommy Fleetwood, former British Open champion Henrik Stenson, and Olympic gold medallist Justin Rose.