The two boxing superstars face each other on May 2 at the MGM Grand in Las Vegas.
Pacquiao's trainer, Roach, feels that coming second means nothing as the world only remembers winners.
In a chat with Rolling Stone, Roach reiterated his feeling on how important it was to win the fight for his man.
"I feel we have to win this fight. This is a must-win situation.
"We don't want to be second best. Second best no one remembers," Roach told Rolling Stone.
Roach confirms that he respects Mayweather as a fighter but not as a person and keeps telling Pacquiao that.
"He's a bad guy," Roach reportedly tells Pacquiao. "He's bad for boxing. You'll be doing a courtesy to the world if you knock him out.
"He knows he's a bad person."