Yordenis Ugas has beaten Manny Pacquiao by unanimous decision, putting on an impressive technical performance on 11 days' notice and retaining his WBA welterweight title.

Ugas (27-4) seized his opportunity as the late injury replacement for Errol Spence Jr, frustrating Pacquiao (67-8-2) throughout what might be the final fight of the 42-year-old Filipino senator's career.

The 35-year-old Ugas threw far fewer punches than Pacquiao, but his blows were more precise and more effective.

Pacquiao struggled to get inside on Ugas' effective jab, while Ugas landed his right hand to increasing effect in the later rounds.

Two judges scored it 116-112 for Ugas, and a third had it 115-113.

After an unimpressive performance in his return from the longest layoff of his quarter-century in professional boxing, Pacquiao said he hasn't decided whether he will fight again.

He also wouldn't confirm whether he will enter the Philippines' presidential race, as is widely expected. He intends to make an announcement next month.

"Let me rest first before my family and I make a decision," Pacquiao said.

The victory was the culmination of a lengthy journey for Ugas, who defected from Cuba two years after winning a bronze medal in the Beijing Olympics.

Ugas quit boxing for two years midway through the last decade, but recovered his career and capitalised on this unexpected shot against one of the greatest fighters of this era, earning his 12th victory in 13 fights.

"He's a great competitor, but I came in here to show I am the champion of the WBA," Ugas said through a translator.

"A lot of respect for him, but I won this fight."

Ugas got this opportunity only last week when Spence was forced to drop out after discovering he had a torn retina during a pre-fight physical.

Ugas had been booked on the undercard, but he seized the opportunity for the spotlight and payday that had been just out of reach since he left Cuba on a small boat bound for Mexico 11 years ago.

T-Mobile Arena appeared to be essentially sold out despite the late opponent change, and the crowd of 17,438 was vocally behind its Filipino hero.

Even after a lengthy layoff in the last stages of his career, Pacquiao remains a surefire draw and a bankable star in a sport lacking both at its highest levels.

Ugas had a clear game plan despite the short notice, working hard in the early rounds with an effective jab and body shots.

Pacquiao was more aggressive and occasionally got the crowd to its feet with combinations, but he didn't immediately find a consistent way to get inside the jab.

Ugas' confidence grew in the middle rounds, and he responded to some action at the seventh-round bell with a defiant shimmy-shake of his shoulders in Pacquiao's direction.

Pacquiao threw roughly twice as many punches as Ugas for long stretches, but they landed about the same number as Ugas showed off his defence and precision against Pacquiao's activity.

Pacquiao caught Ugas with a combination in the 10th round and knocked him back as the crowd rose in excitement, but Ugas recovered and rallied with big shots.