Filipino Manny Pacquiao said a shoulder injury hampered his bid to hand Floyd Mayweather a first ever defeat as a professional on Saturday as the American improved his record to 48-0 with a unanimous decision.

Pacquiao, a 2-1 underdog going into the welterweight bout billed as the "Fight of the Century" at the MGM Grand Garden Arena, suffered the injury during a sparring session, according to his trainer Freddie Roach.

"In the third round, I felt pain in the shoulder," eight-division world champion Pacquiao told reporters after dropping to 57-6-2. "We didn't throw a lot of combinations because it hurt."

"The thing is, what we wanted to do we could not do because of my (right) shoulder."

Roach said the injury occurred after Pacquiao collided with another fighter during sparring and their arms got entangled.

An MRI exam subsequently revealed a tear in the Filipino southpaw's right shoulder, though Roach said the boxer had since made good progress and he felt that he could still compete in the ring.

"You guys saw the fight yourself. When you review the film, you'll see how infrequently he threw the right," said Pacquiao's promoter Bob Arum.

While Mayweather landed 148-of-435 punches during the 12-round four-title unification bout, Pacquiao connected with just 81-of-429 as his all-out, attacking style was neutralised by the American's brilliant defence.

"He was moving around too much," said eight-division world champion Pacquiao, who usually relies on his lightning-fast hand and foot speed in the ring.

"It wasn't easy throwing punches at him. If he would have stayed in one place, then I could have thrown punches.

"I was cutting him off and countering. I wanted to fight. I was very surprised at the score. I had no problem handling his power. I thought I was up in the fight, so that's why I didn't attack harder in the 11th and 12th rounds."

Like Pacquiao, Roach thought they had done enough to win the fight.

"I thought we pulled it out," said the shrewd trainer. "We pressed the action. I asked Manny for more combinations between the round. I thought he fought flat-footed too many times."