Manny Pacquiao is expected to undergo surgery later this week for a rotator cuff tear that he failed to disclose before he lost a highly anticipated boxing match against Floyd Mayweather Jr. in Las Vegas Saturday night.

“We have an MRI scan that confirms he has a rotator cuff tear. He has a significant tear,” Dr. Neal ElAttrache told ESPN after the fight. ElAttrache said the injury will sideline the 36-year-old eight-division world champion for between nine and 12 months.

Meanwhile, Nevada boxing regulators are also looking into possible disciplinary action against Pacquiao for failing to disclose the injury he suffered while training for the fight. His trainer, Freddie Roach, said Pacquiao was injured in April, and that he appeared to be healed. Pacquiao claims to have reinjured his shoulder in the fourth round after landing hard punches against Mayweather.

Pacquiao’s handlers unsuccessfully sought to get a pain-killing shot in the shoulder for him in the dressing room before the fight.

Dr. David Teuscher, president of the American Academy of Orthopaedic Surgeons and a sports injury specialist who has not treated Pacquiao, told FoxNews.com that the shot likely would have helped with acute inflammatory pain and could have provided some discomfort relief.

“I think both he and his trainer, and the public, saw that he had altered his fighting style, and when you are in pain, you will change how you use that arm and that shoulder,” Teuscher, who is based in Texas, said.

“In boxing, you have to keep your hands up in front of your face and upper torso to defend the blows, or hold them out in a defensive position, and all of that dramatically affects the rotator cuff tendons,” Teuscher said, adding that the sport of boxing puts a large amount of stress on the rotator cuff area.

“It’s his punching shoulder, not his jab shoulder, but either one you have to be able to hit with both, which means you have to be able to extend it out there,” he said.

Though it is not known which tendon exactly was torn, or the extent of the injury ahead of the fight, ElAttrache’s description of a “significant tear” leads Teuscher to believe it is a full-thickness tear, which requires surgical treatment, and would not have healed in the time period leading up to the fight as Roach had indicated.

“A full-thickness tear is not going to heal on its own, it may settle down, it may feel more comfortable, but it may have also been subject to micro re-injury,” Teuscher said. 

A tear occurs when one or more of the rotator cuff tendons becomes detached from the humerus bone. When a muscle is pulling in the opposite direction of the attachment point, it may, overtime, cause a small tear to become bigger, and a bigger tear to become irreparable.

“It’s not necessary for everyone to have it repaired, but for a young athletic man like him, if he has any chance to get back into boxing, he’s probably making the right decision,” Teuscher said.

The surgery required involves a biologic process which is why a prolonged recovery period is needed for rotator cuff tear patients.

“What has to happen is the tendon has to be reattached to the bone, typically we use non-absorbable or absorbable suture anchors into the humerus bone, and [that] ties the tendon back to the bone,” Teuscher said. “But that won’t hold forever, it has to have a biologic process and undergo a strengthening period.”

Teuscher said that patients usually face three phases of recovery post-surgery. The first twelve weeks are spent mostly in an immobilizer, with patients completing periodic passive stretching. The second twelve weeks include various active motions where the arm is moved under its own power, with some resistance range of motion activities, but the patient must continue to avoid certain activities and positions.

For typical patients, they can usually return to work and resume normal activities in the third phase, but for Pacquiao, Teuscher said, this phase may take another 3-6 months to complete as he’ll have to concentrate on returning to athletic physical shape and his activities go beyond a normal day-to-day schedule. He'll also need to take this time to listen to his trainers and physicians to limit his odds of re-injuring his arm. 

“It’s not terribly common, but if they're not compliant it becomes increasingly more common, because the body only heals one way, and you can’t speed up the clock,” he said.

As far as Pacquaio’s future in boxing, Teuscher said it's in his best interest to begin with sparring before seeking a rematch with Mayweather. 

"I think the jury is out as to how well he will be able to recover and come back," he said. "He is going to want to have some sparring with some lesser opponents and build up to it."