Ronnie Shields, who is preparing Juan Lazcano to fight Hatton on Saturday, said his diet of takeaways and alcohol is finally taking its toll.
The bout at the City of Manchester stadium is Hatton's first since his loss to Floyd Mayweather in December.
"I think his lifestyle has finally caught up with him," said Shields.
"Ricky has a lot to prove because he got knocked out by Floyd Mayweather.
"Floyd's not a puncher, he's never knocked anyone out, he's a decision guy. For him to KO Ricky, it wasn't because of Floyd's punching power, it was because Ricky has gone downhill.
"The Ricky in the Mayweather fight wasn't the Ricky you saw against Kostya Tszyu or the other guys he's beaten.
"I had Ricky ahead after five or six rounds (against Floyd) then all of a sudden he gets hit with shots he normally wouldn't be getting hit with and he gets hurt.
"I think the lifestyle is catching up on him. Whether Ricky realises that or not I don't know."
Hatton stepped up to welterweight to fight Mayweather, regarded as the best pound-for-pound fighter in the world, and blamed the defeat on his failure to execute his game plan.
Saturday's bout sees the Briton drop down to his preferred weight as he attempts to defend his reputation as the world's best light-welterweight.
But veteran American trainer Shields believes Lazcano, 33, has the potential to cause an upset in front of an estimated 55,000 fans.
"Juan is a veteran of the boxing game, one of the best fighters I've seen in a long time," he said. "I've never seen him more hyped for a fight and I've been around him for 20 years."
"I used him as a sparring partner for Pernell Whitaker and Zab Judah and he has always been able to handle himself."
Although his trainer believes Hatton is past his best, Lazcano has praised the Manchester-born fighter's spirit.
"I tip my hat to Ricky," said the Mexican. "He's stepping out so soon after his last fight and I really have to commend him for that.
"He's not your ordinary fighter. He's proved he's got a lot of 'cojones' to do what he's doing."