Aug 10 (Reuters) - Welterweight champion Errol Spence, Jr. has been forced to withdraw from his Aug. 21 fight against Manny Pacquiao due to a retinal tear in his left eye, organisers said on Tuesday. Cuban Yordenis Ugas, who won a bronze medal at the 2008 Beijing Olympics, has agreed to replace Spence in the main event at T-Mobile Arena in Las Vegas and defend his WBA welterweight belt against Pacquiao, who has won titles in eight divisions. Spence, the WBC and IBF welterweight world champion who has a 27-0 record, learned he had a torn retina during a pre-fight medical examination by the Nevada State Athletic Commission in Las Vegas on Monday. "I was excited about the fight and the event," Spence said in a news release. "Unfortunately, the doctors found a tear in my left eye and said I needed to get surgery on it ASAP and that there was no way I could fight with my eye in that condition." Spence, who has returned home to Dallas, Texas and will have surgery to repair the tear on Wednesday, is expected to make a full recovery and return to the ring. "I ask everyone to join me in praying for a full and complete recovery for Errol Spence Jr.," said Pacquiao. "Thank God his physical examination discovered his eye condition before he suffered any further damage." In Pacquiao"s most recent fight, in July 2019, the Filipino southpaw beat Keith Thurman in a split decision to win his since-vacated WBA super welterweight title belt. Ugas (26-4, 12 KOs) has established himself as an elite welterweight since returning from a layoff in 2016 and was elevated to world champion in January after winning a WBA championship belt by defeating Abel Ramos last September. The 35-year-old Cuban said it is an honor to get a chance to fight Pacquiao and that he is ready for the challenge despite the short notice. "I have a tremendous amount of respect for Pacquiao, but I am coming to win this fight," said Ugas. "I"ve been in camp working hard with my coach Ismael Salas and I know together we will come up with a masterful game plan to combat anything Manny will bring to the ring."