Pacquiao and Mayweather are set to split upwards of £150m following their welterweight bout at the MGM Grand.
But Freddie Roach said of Mayweather: "I really am wondering if he's going to turn up on the night.
"I don't think any fighter is afraid, but I don't think he wanted this fight. He was forced into a fight he didn't want to take."
Roach added: "I just don't know why Floyd has gone quiet for this fight. His speech is very subdued."
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Five-weight world champion Mayweather, 38, was unmoved by Roach's claims, stating: "I'll be there.
"I talked trash in the past but this fight sells itself, so I don't have to."
Roach said Mayweather's camp had also made a complaint about the protective cup that Pacquiao will use, citing that as further proof of Mayweather's fragile state of mind.
Roach said the Nevada Commission had inspected the cup at Pacquiao's training camp and approved it.
"It's the regular cup," said Roach, a seven-time trainer of the year who was in Oscar de la Hoya's corner when Mayweather beat him in 2007.
Mayweather's trainer, Floyd Sr, also went on the offensive at Wednesday's news conference.
"Manny Pacquiao ain't got it," he said. "He can't punch. It's going to be one-sided and Pacquiao is going to be put to sleep. Good night."
He also mocked Pacquiao and Roach in rhyme, including the line: "When Floyd breaks his jaw, he will take time to recoup. And when it's all finished, he'll be sucking on soup."
Six-weight world champion Pacquiao, 36, said he wanted the fight more than Mayweather and criticised the American - unbeaten in 47 bouts -  for being money-focused.
"It's all business for him," said Pacquiao. "For me, it's about giving the fans who are paying all this money a great fight. They deserve that."
The Philippine legend added he was in good shape for the fight, which is the richest in history and also for the WBC, WBA and WBO titles.
"The feeling I had three years ago, 10 years ago, is back," he said. "I haven't felt like this in my last few fights, I feel different. I'm relaxed and confident."
Manny Pacquiao's trainer says he fears Floyd Mayweather might not turn up for Saturday's super-fight in Las Vegas.