"What (Ariza) doesn't realize is that Manny hits me ten times harder every day. It was like a girl kicking me," Roach said.
Sunday's fight (Saturday night U.S. time with an undercard beginning at 7 p.m. ET) will be Pacquiao's first since getting savagely knocked out by Juan Manuel Marquez almost a year ago.
The Filipino, once regarded as pound-for-pound the best fighter in the world, is keen to restore his standing in the boxing world after successive defeats against Marquez and a much-criticized judging decision in favor of Tim Bradley.
"He's hungry after the year off, he's anxious to get in the ring and he's anxious to show the world he's still an elite fighter," Roach said. "The best way to do that is to knock Brandon Rios out, we both know that, and I have stressed that to him.
"Initially I didn't think the fight would go six rounds, and after the way he's worked out, I don't think it will go four."
Pacquiao has not stopped an opponent since he fought Miguel Cotto four years ago, but Roach feels Rios' aggressive style will open the door for a quick finish.
"Rios is a tough guy and tough guys don't win fights, good boxers win fights," Roach said. "The more I watch him on tape the worse he gets.
"We will box him and the knockout will come with Manny's hand speed and combinations and Rios not having a good defense. One thing we know is that Rios will fight back and that's our big advantage because Manny is a beautiful counter-puncher and we've been working quite a bit on that in the gym."
Sunday's fight in Macao, which will be Saturday night U.S. time, is for the vacant WBO welterweight title.