Manny Pacquiao's ring return at the age of 42 ended in defeat as Yordenis Ugas outpointed the Filipino icon in Las Vegas.
'Pacman' - who was fighting for the first time in two years - blamed leg cramps for his performance, stating he was unable to move around the ring like he would have liked.
The eight-weight ruler is now questioning his future in the sport with retirement on the cards as he bids to become president next year in his native.
Ugas - now with a record of 27-4 - took the fight on 11 days' notice after Errol Spence Jr. pulled out of the fight with an eye injury.
It was Ugas' first defence of the title having defeated Abel Ramos for the vacant WBA title - a belt which Pacquiao was stripped of for inactivity.
The Cuban fighter - who had originally been preparing to take on Fabian Maidana - counterpunched his opponent throughout the contest and retained his WBA welterweight title in front of a partizan Pacquiao crowd at T-Mobile Arena.
The scorecards read 115-113, 116-112, 116-112 in favour of Ugas.
Pacquiao hints at retirement
"I'm sorry that we lost tonight, but I did my best," Pacquiao said. "I don't know, I don't know. Let me rest first and relax [before] I decide whether I will fight or not.
"I will make a final announcement (about running for president) next month. I know that I am facing a big problem that's more difficult than boxing but I want to help the people. I want to help them.
"In the future, you might not see Manny Pacquiao again to fight in the ring. I don't know. But for how many decades, I'm so happy with what I've done in boxing. I contributed a record and brought boxing to the top, and especially, brought honour to my country, the Filipino people.
"I'm not saying this is my excuse, but my two legs were cramping. That's why I could not move around. In the early days (of my career), I could easily move and outbox him," Pacquiao continued.
"But this time around, it was like my two legs were tight and hurting me in the second round until the 12th round.
"But I'm not making excuses. I tried to counter his punches. That's boxing. I have done a lot in boxing and I'm happy I'm safe.
"I ran in the mountains and also did 32 rounds [of sparring] every day so I don't know. We are not young anymore," Pacquiao added.
"I'm very excited but most of all, I want to thank Manny Pacquiao for giving me this moment in the ring today," Ugas said.
"I told you that I'm the only champion of the WBA. He's a great competitor and I have a lot of respect for him but I won the fight.
"The double jab was the lead punch. We only had two weeks of training but I listened to my corner and it all worked out," Ugas added.
Ugas could now be in line to take on Spence in a welterweight unification next if the American can recover from a detached retina he suffered in training.
It is unknown as to how long Spence will be out for but he holds the WBC and IBF world title versions at 147lbs while Terence Crawford is the WBO champion.