The Saudi Pro League side will take on Al Shorta in the first round of the competition on Monday as they look to impress in another competition after their indifferent start to league action.
Ronaldo and Co are currently sixth in the Saudi Pro League table, with one win and two draws to their names so far.
After drawing their first match of the campaign, they beat Al Feiha 4-1 in their second match, with Ronaldo scoring the second goal of the game and his second of the season.
If they are to get off to a good start in their continental competition, however, they will have to do so without their star man, and a statement was released surrounding his fitness on Sunday.
His club, however, have issued a statement addressing his fitness ahead of Monday's match-up
'Al-Nassr captain Cristiano Ronaldo wasn't feeling well today and was diagnosed with a viral infection,' the statement read. 'Team's doctor confirmed he needs to rest and star at his place.
'As a result he will not be travelling with the team to Iraq today. We wish a speedy recovery to our captain.'
Al-Nassr also have the likes of Aymeric Laporte, Sadio Mane and Marcelo Brozovic in their ranks.
In a recent interview, Ronaldo claimed that United need to 'rebuild' and questioned Ten Hag's decision to say the club cannot compete for the league.
Ronaldo's second spell at United came to an end in 2022 following a disagreement with Ten Hag.
The Portuguese star reflected on his departure in an interview with Rio Ferdinand.
Ronaldo's comments read: 'Man United coach can NOT say they can't compete to win the Premier League and the Champions League every year.
'This is Man United! You have to be there, to mentally say... listen, maybe we don't have that potential, but I cannot say that. We're going to try. You have to try.