Subkhiddin Mohd Salleh, a FIFA referee between 2000 until his retirement in 2011, said he carefully watched the video of the clash between Malaysian defender Azam Azmi and Vietnamese defender Doan Van Hau in the 59th minute of the match in Hanoi.
Commenting on Malaysian television channel Astro Arena, he said he divided the situation into two phases: Hau's push that caused Azmi to fall and then the counter-attack by the latter.
"When Azmi struck back, the ball was still rolling, and therefore the referee can give Vietnam a penalty according to the rules.
"But in my opinion, the first thing that needs to be considered is Hau's fault and [only] then Azmi's fault."
If the referee thought Azmi had retaliated against Hau, the Malaysian defender deserved a red card, he said.
"But the match should have continued with a free kick for Malaysia, not a penalty for Vietnam, because Van Hau pushed Azmi first.
"The Malaysian defender struck back unnecessarily, but [it appears] referee Ryuji Sato had ignored Hau's fault."
Salleh, 56, broke into the "AFC Elite" group of Asia's top referees in 2008.
He was on duty at the 2010 World Cup as the fourth referee and was the chief referee at several matches during the 2011 Asian Cup.
After retiring he worked as a FIFA referee instructor.
Sato has been a FIFA referee since 2009. He has refereed in Asian Cups, the 2015 U20 World Cup, the 2021 FIFA Arab Cup, AFC Champions Leagues, and the 2016 Olympic Games.
In the match on Tuesday he also gave Vietnamese striker Van Toan a second yellow card and failed to spot Hau elbowing his opponents on a few occasions.
Despite playing with 10 men, Vietnam won 3-0 to go to the top of group B.