Trainer Freddie Roach played down hopes of a Manny Pacquiao showdown with Floyd Mayweather and said unbeaten two-belt world champion Danny Garcia could be next for "Pacman" after he fights Chris Algieri on Sunday.

The drop to light welterweight (140lb) would be a further blow to prospects of a $1-billion megafight with the unbeaten Mayweather, whose last three contests have been at light middleweight (154lb).

Roach instead is targeting the impressive Garcia (29-0, 17 KOs) who holds both the World Boxing Council and World Boxing Association belts at light welterweight.

"I've been waiting three-and-a-half years for this (Mayweather) fight. I don't think it's going to happen," Roach told reporters on Friday at the Venetian Macau.

"(Promoter Bob) Arum says it might happen but can they deliver Mayweather? I don't think so," added the Hall of Fame trainer, who has guided "Pacman" since 2002.

"You know, if it was going to happen I think it would have happened by now. I want it to happen as badly as you guys."

Pacquiao defends his World Boxing Organization welterweight (147lb) crown on Sunday morning (Saturday night US time) against American Algieri at an agreed catchweight limit of 144lb.

"If Manny feels good in this fight at 144 then I think his next fight will be at 140 because he makes 140 very easily," said Roach.

"First fight back? Danny Garcia. That's my personal opinion."

The fear is that Sunday's clash with the largely unknown WBO light welterweight champion Algieri will not attract the lucrative pay-per-view figures in the United States of previous Pacquiao contests.

There is a dearth of truly world-class opponents in the 147lb division, making the explosive Garcia - who has recorded fourth-round knockouts against both Britain's former world champion Amir Khan and Mexican great Erik Morales - the most attractive alternative to the elusive Mayweather.

"Danny Garcia at 140, we'll go there," added Roach. "There are options out there. Mayweather's not everything.

"I think Manny's a better fighter at 140. He's faster and more explosive - I think the knockouts will come back."

Roach hinted that Pacquiao could drop further, maybe back to lightweight (135lb) if the right opportunity came up.

"Yes he could make 135. He's been in the 130s a couple of times during training camp already," Roach revealed.

"When he gets a little too low in weight I send him home and tell him to have a good steak."