Three days after his upset loss, Pacquiao released a statement in his capacity as a Philippines Senator, backing calls from the country's Games and Amusement Board (GAB) for a review by the World Boxing Organization (WBO) into the refereeing and judging of the contest.
"WBO should take appropriate action on the letter sent by the Games and Amusement Board (GAB) so as not to erode the people's interest in boxing," Pacquiao's statement reads.
Pacquiao lost his WBO Welterweight title in the fight dubbed the "Battle of Brisbane" after judges unanimously granted the win to relative newcomer Horn.
In his statement, Pacquiao said he accepted the decision "but as a leader and (a) fighter I have the moral obligation to uphold sportsmanship, truth and fairness in the eyes of the public.
"I love boxing and I don't wanna see it dying because of unfair decision and officiating."
In his prime, the Filipino boxer, (59-7-2, 38 KOs), was considered by Ring magazine to be the best pound-for-pound boxer in the world
The original letter from the GAB to the WBO said: "While we respect the decision, we are constrained to request for a thorough review by the WBO for possible miscalls of the referee where some deductions were not made and of the judges in their judging that may gave caused varying opinions on their objectivity."
CNN has not been able to reach Horn's representation for comment.
The result has been called one of the greatest upsets in the sport's history.
It sparked a flurry of social media protest from boxing luminaries and fans alike, who perceived a bias towards the Australian fighter in both refereeing calls and the ultimate decision of the judges, who awarded the fight to Horn 117-111, 115-113, 115-113.
Three-time world heavyweight champion Lennox Lewis disputed the result on Twitter: "This is what's wrong with boxing. Horn was very game but I'm hard pressed to see how he could have won that fight by any stretch!"
Despite the outcry over the decision, Pacquiao was gracious in defeat.
"I didn't expect [him to be] that tough," he said after the fight. "It's OK. Part of the game. That's the decision of the judges. I respect that."
Before the bout, few thought Horn could go the distance against the 38-year-old Pacquiao, whose last fight and win was against American Jessie Vargas in November 2016.