Manny Pacquiao cast a shadow over Floyd Mayweather's conclusive points victory in Las Vegas by claiming he considered postponing the welterweight showdown because of a shoulder injury.

Pacquiao suffered a tear in the joint during sparring around three weeks ago and requested permission from the Nevada State Athletic Commission for a pain-killing injection, but the request was refused.

Apart from a dominant fourth round when he backed Mayweather up with a hard southpaw left before unleashing a barrage of punches, he made little impression on the richest fight in history.

Pacquiao revealed at the post-fight press conference that he re-injured his shoulder early in the clash at the MGM Grand and that prevented him from throwing punches with his right hand.

“We were planning to postpone because for two weeks I didn’t train well. We couldn’t do what we wanted to because of my shoulder,” Pacquiao said.

“From the third round I felt the pain in my shoulder. When I threw a lot of punches it hurt, so I backed off.

“I wanted to be more aggressive but I could only use my left hand. It’s difficult to fight with one hand. Even though I hurt my shoulder, I didn’t complain in the ring. It’s part of the game.

“We planned to file with the commission a request for a shot to numb my shoulder, but they did not allow that so we didn’t get a shot. We respect that.

“Two weeks before the fight, one week before the fight, the shoulder was getting better and better but wasn’t totally 100 per cent. I couldn’t use it because there was something wrong.”

On first glance Mayweather’s dominance in the 48th victory of a flawless record would have made a September rematch in the final fight of his career pointless, but Pacquiao’s revelations about his injury may now make that scenario possible.

The 38-year-old Las Vegas resident offered Pacquiao little sympathy and suggested he should not have disclosed the damage to his shoulder.

“I had injuries also going into this fight and if he had come out victorious, I would have shown respect and said he was the better man,” Mayweather said.

“Both of my arms and hands were injured, but I always find a way to win.”

As Pacquiao's camp assembled for the press conference, promoter Bob Arum could be heard saying "let's not make a big deal out of it", only for trainer Freddie Roach to point out that the eight-weight world champion did not have use of one arm.

Arum was forced to defend the decision to stage the showdown even though Pacquiao’s fitness was in doubt given the extraordinary sums being paid by fans for live tickets, closed circuit access and pay per view.

“Athletes always fight hurt. We felt that the work that was done on the shoulder in training would give him the opportunity to use the right hand,” Arum said.

“We were disappointed when it kicked up again in the third round, but this is always the case in sport.

“A guy is injured in training, he thinks he’s conquered it and dealt with it, and then he gets re-injured. It happens in football and every sport.”