Manny Pacquiao could face disciplinary action from Nevada boxing officials for failing to disclose a shoulder injury before his fight with Floyd Mayweather Jr.

The Nevada state attorney general's office is to investigate why Pacquiao ticked 'no' a day before the fight on a commission questionnaire asking if he had a shoulder injury.

"We will gather all the facts and follow the circumstances," said Nevada Athletic Commission chairman Francisco Aguilar on Monday. "You want to make sure fighters are giving you up-to-date information."

Pacquiao could be fined or suspended for not answering the question accurately on the form he filled out himself just before Friday's weigh-in. He went on to lose a unanimous points decision to Mayweather in the richest fight ever.

He is also expected to undergo surgery later this week to repair a "significant tear" in his rotator cuff (shoulder muscles), according to Los Angeles orthopaedic surgeon Dr Neal ElAttrache, who examined Pacquiao on Monday.

Pacquiao's promoter said in a statement late on Monday afternoon that the injury was disclosed to the US Anti-Doping Agency, which approved the use of an anti-inflammatory shot for the fight.

But USADA was only a third party to the fight, charged only with testing the fighters for banned substances in training and the night of the bout.

"We had no medical information, no MRIs, no documents," said Travis Tygart, who heads the USADA. "It was not an anti-doping issue.

"The real question is why his camp checked 'no' on the disclosure. Either they made a terrible mistake to not follow the rules or they were trying not to give information to the other side. I'm not sure there's a middle ground."

Tygart said his agency, which was hired by promoters to oversee drug testing for the bout, was contacted on April 7 asking about the use of various substances and whether they were allowed under anti-doping rules.

He said there was another call 10 days later asking about using a different substance, again for what the USADA was told was an unspecified shoulder problem.

A little more than two hours before the fight, Pacquiao's corner asked Nevada regulators if he could be given a shot of Toradol, an anti-inflammatory.

Aguilar refused permission, saying the commission had no previous indication there was an injury, it was too late to seek proof of it and, in fairness to the Mayweather camp, could not allow an injection.

"Our job is to protect the health and safety of fighters and the integrity of the sport," Aguilar said. "We expect our fighters to be forthright."

After the fight, Pacquiao said his shoulder had improved enough for him to go into the ring and said it had not bothered him until the fourth round, when he hit Mayweather with a big left hand and went after him with a series of punches.

Pacquiao's trainer, Freddie Roach, said: "We thought at one time we'd postpone the fight, but as the weeks went on it was getting better. I thought the progress was good enough."

Nevada Athletic Commission executive director Bob Bennett said: "It's not just the fact he didn't fill out the question completely, it was that he wasn't honest and they didn't tell us a month ago when he had the shoulder injury.

"They're not obligated to, but two hours before the fight they wanted a shot that's a pain killer in essence. That put us in a very precarious position."