Cristiano Ronaldo was diagnosed on Sunday with a viral infection and will miss Al Nassr's Asian Champions League opener this week at Iraq's Al Shorta.
The Portuguese veteran is targeting Asia's top club crown to add to his long list of achievements when the competition kicks off on Monday.
"Al Nassr captain Cristiano Ronaldo was not feeling well today and was diagnosed with a viral infection," said a club statement posted on social media platform X on Sunday.
"The team's doctor confirmed he needs to rest and stay at home. As a result, he will not be travelling with the team to Iraq today. We wish our captain a speedy recovery."
The former Real Madrid and Manchester United forward saw his first attempt with the Saudi side Al Nassr end in a quarterfinal penalty shootout defeat to eventual champions Al Ain earlier this year.
Now rebranded as the Asian Champions League Elite, Al Nassr and the other teams from Saudi Arabia will be a major threat after spending vast sums of money in the last two years on the likes of Ronaldo and Neymar.
England forward Ivan Toney was the latest big name to swap Europe for Saudi when he left Brentford for Al Ahli last month for more than $45 million.
As part of the new-look competition, Saudi Arabia – which is expected to host the 2034 World Cup – will stage a mini-knockout tournament from the quarterfinals onward to decide the champions, who will take home at least $12 million.
Al Nassr, Al Ahli and Neymar's Al Hilal will all be involved when the Champions League Elite launches on Monday with a new format that splits 24 teams evenly into two groups of East and West.
Each team will play eight different opponents in the group stage.
The top eight teams from each zone will play in the last 16 over two legs in March, before the action moves to the final stages in Saudi Arabia from 25 April to 4 May.
INJURED NEYMAR MISSING
Saudi champions Al Hilal are looking to win a record-extending fifth Asian title but they are still waiting for the return of Brazilian ace Neymar, who has been sidelined since suffering a serious knee injury last year.
Al Hilal have signed Portugal full-back Joao Cancelo from Manchester City, bolstering a squad that already featured Aleksandar Mitrovic, Sergej Milinkovic-Savic and Ruben Neves.
Despite the absence of the 39-year-old Ronaldo for Monday's opening encounter in Bagdad, Al Hilal are still able to call on Sadio Mane, Aymeric Laporte and Marcelo Brozovic.
The defending champions, Al Ain, are from the United Arab Emirates and were coached by Argentina great Hernan Crespo to victory against Japan's Yokohama F-Marinos over two legs in May's final.
Al Ain are one of two UAE clubs taking part in this year's edition, with teams from Qatar, Iran, Uzbekistan and Iraq completing the West zone line-up.
In the East, clubs from Japan have posted the best results in recent years and Yokohama return to the Champions League to spearhead their challenge.
Coach Harry Kewell has been sacked since leading them to last season's final, with the former Liverpool and Leeds forward paying the price for his team's lacklustre domestic form.
Kawasaki Frontale and Vissel Kobe are also representing Japan, while three-time champions Pohang Steelers, twice winners Ulsan and debutants Gwangju will carry the flag for South Korea.
Chinese clubs' spending power has long since dried up, but former Chelsea attacker Oscar is still on the books of Shanghai Port, who are led by Australian coach Kevin Muscat.
Shandong Taishan and Shanghai Shenhua are also involved from China, while one club each from Australia, Thailand and Malaysia feature in the East league stage.