Put this on the long list of tactics that underdog Manny Pacquiao must be wary of when he faces undefeated Floyd Mayweather Jr.: trash-talking.

“Manny has not been in with a lot of trash-talkers,” said Freddie Roach, Pacquiao’s trainer. “Mayweather is like Ali in the sense that he will talk to you a lot in the fight. He will rest on the ropes, chatter in your ear and try to get you out of your game.

“I’m...