The four-time major winner has been ruled out of this week's Abu Dhabi Championship after scans revealed he has a stress fracture in one of his ribs.
McIlroy had played with a suspected back injury at the South African Open, where he was beaten in a playoff by world No. 251 Graeme Storm of England on Sunday.
"It's bitterly disappointing to have to withdraw from the Abu Dhabi HSBC Championship," McIlroy said in a statement Monday.
"I think everyone knows how much I love playing this tournament, which is one of the best on the European Tour. To be forced to miss this week through injury is really quite annoying to be perfectly honest, but I am sure the tournament will be a huge success and I hope everyone involved has a great week.
"In situations like this you simply have to listen to the experts, and the team I have consulted have all advised me to rest until my rib has fully recovered."
McIlroy last topped the world rankings in September 2015, but had hoped he would move above Jason Day with victory in Abu Dhabi -- a tournament where he has been runner-up four times in the past six years.
Day didn't play on the PGA Tour last week, having tied for 13th at the Tournament of Champions in Hawaii to start his 2017 schedule.
While Storm climbed 118 places in the rankings after the second European Tour win of his career, the other significant mover was Justin Thomas.
The American capped a perfect week by claiming his fourth PGA Tour title in record-breaking style and breaking into the top 10 of the world rankings, taking eighth place.
Thomas followed up his Tournament of Champions victory by winning the Sony Open -- also in Hawaii -- by seven shots from Justin Rose.
The 23-year-old started by becoming the youngest player to shoot 59 on Thursday, then added a 36-hole scoring record and finished by setting the best for 72 holes.
His 27-under-par total of 253 was seven strokes clear of Olympic champion Justin Rose, with world No. 5 Jordan Spieth third on 19 under.