Manny Pacquiao will fight Yordenis Ugas later this month after WBC and IBF welterweight champion Errol Spence Jr was forced to pull out of their fight with an eye injury.
Eight-division world champion Pacquiao and American Spence Jr were due to fight in Las Vegas on 21 August.
But a medical revealed Spence has a torn retina and needs surgery.
Spence said he was excited about the fight but "there was no way I could fight with my eye in that condition".
WBA welterweight champion Ugas is promoted from the undercard to face Pacquiao instead.
Pacquiao has not fought since he beat Keith Thurman for the WBA welterweight title in July 2019.
The 42-year-old, who is a serving senator in his native Philippines, was stripped of the title in January because of inactivity.
Ugas beat Abel Ramos for the vacant WBA regular welterweight title in September before being elevated to WBA super champion after Pacquiao was stripped of the belt.
The 35-year-old Cuban had been preparing to make his first title defence against Fabian Maidana, but will instead take on Pacquiao, who has a 62-7-2 record.
"First and foremost, I ask everyone to join me in praying for a full and complete recovery for Errol Spence Jr," Pacquiao said in a statement.
"Thank God his physical examination discovered his eye condition before he suffered any further damage.
"I have agreed to fight Yordenis Ugas on August 21 for the WBA welterweight super championship. The proper way and the only way to win a world title is inside the ring."