Al Nassr faltered in Cristiano Ronaldo's absence on Monday, kicking off the revamped Asian Champions League with a 1-1 draw against Al Shorta in Baghdad.
The Saudi club's captain, Ronaldo, missed the match due to a viral infection, as announced by the team a day earlier.
Despite a promising start for Al Nassr, who were eliminated by eventual champions Al Ain in last season's quarterfinals, they managed to take the lead early on.
Full-back Sultan Al Ghannam capitalized on Otavio's precise, first-time pass in the 14th minute, sliding the ball past Al Shorta goalkeeper Ahmed Basil.
However, the hosts drew level 10 minutes later. A mix-up at the back between new signings Salem Al Najdi and former RB Leipzig defender Mohamed Simakan allowed Al Shorta forward Mohammed Dawood to steal in and send a superb effort high past Bento in the Al Nassr goal.
Meanwhile, fellow Saudi side Al Ahli eked out a 1-0 victory at home against Iran's Persepolis in Jeddah, with new recruit Ivan Toney making his Asian football debut.
Ivory Coast international Franck Kessie set the home side on their way in the second minute, rifling home from Riyad Mahrez's assist.
Persepolis were awarded a penalty four minutes before halftime when Merih Demiral was deemed to have committed a foul inside the area; however, the decision was overturned following a VAR review.
In the second half, Toney won Al Ahli a spot-kick of their own when he was fouled by rival defender Hossein Kanani, but Mahrez's penalty was well saved by Persepolis goalkeeper Alexis Guendouz.
England international Toney, a high-profile signing from Premier League club Brentford late last month, missed a golden opportunity to open his account five minutes from time but dragged his shot wide.
He had made his debut for Al Ahli last Friday.
Elsewhere, Al Ain began their title defense with a 1-1 draw at home against Qatar's Al Sadd.
Akram Afif, 2019 Asian Footballer of the Year and player of the tournament at the Asian Cup earlier this year, opened the scoring at Hazza bin Zayed Stadium on the hour.
But Al Ain earned a point 10 minutes from time through Argentine Matias Palacios.
In Monday's other match, Iran's Esteghlal defeated Qatari side Al Gharafa 3-0.
The new format features 24 teams – 12 from West Asia and 12 from East Asia. Each club will play four matches at home and four away, with the top eight in each side of the draw advancing to the knockout stages early next year.
