The flamboyant Roach backed Pacquiao for a fifth or sixth-round knockout of the American in their November 24 World Boxing Organisation welterweight title bout in Macau.
But if the fight does not go according to plan, he said he would have no hesitation in telling Pacquiao to quit the sport after what would be his third straight defeat.
And Roach, a former boxer who suffers from Parkinson’s disease, said they had an “agreement” that Pacquiao would retire if advised to by his trainer.
“If I see he is slowing down and slipping in the fight I will be the first one to tell him,” Roach told journalists via conference call from Pacquiao’s training camp in General Santos City, the Philippines, late on Wednesday.
“We have an agreement that I will tell him that and he will retire. I don’t see him slipping in the gym at this moment and he’s doing really well. He’s fired up and anxious to get back in the ring and I see good things.
“If things don’t go well we will talk about retirement and going into politics full-time or something like that.”
Pacquiao, who is nearly 35 and was regarded as one of the best pound-for-pound boxers of his generation when he won world titles in eight weight divisions, has not fought in nearly a year since he was knocked out by Mexico’s Juan Manuel Marquez in the sixth round in Las Vegas last December.
It was Pacquiao’s second consecutive defeat after he lost a controversial split decision to Timothy Bradley six months earlier — his first loss in seven years.
Pacquiao’s build-up to the “must-win” fight with Rios has been disrupted by super typhoon Haiyan, which killed thousands of people and left hundreds of thousands homeless when it swept through central regions of the Philippines.
But Roach said he had told Pacquiao, who is a member of congress, that it was impossible to take time out of training to visit survivors and survey aid efforts in the affected areas.
“He told me he wanted to go down on Sunday but it’s too close to the fight for him to take any time off and we talked about that,” Roach said. “He is focused on the fight still but obviously it is a big distraction because it killed all of those people.
“We do talk about it in the gym — about how many people got killed in the storm and how many more have been affected. He is concerned about it yes, very much, but I think we have him pretty much on track on the fight.
“He knows it’s a big fight and he knows it’s a must-win situation and it’s bigger than that because he has to win for the country also, not just his boxing career, he knows he has to win for the people and he told me that yesterday.
“They seem to be inspiring each other.”
Roach added that Pacman was “90 percent ready” to face “fat guy” Rios, who is seven years younger and 1.5 inches taller than the five foot 6.5 inches (1.69 metres) Filipino southpaw.
He said if Pacquiao wins in Macau, they will seek a rematch with Marquez as a priority ahead of the long-awaited match-up with undefeated American Floyd Mayweather.
“We want Marquez one more time, yes. That’s the fight we want,” he said. “Mayweather too of course, but Marquez, he got lucky and we want to take that back.”
Pacquiao wraps up his training camp and departs for Macau on Monday, ahead of the fight which will be held next Sunday morning local time so as to be live for Saturday evening TV audiences in the United States.