Andy Ruiz Jr "lacks in desire" and "finds an excuse not to do something", according to his former trainer Abel Sanchez.
Sanchez trained Ruiz for a short period and was in his corner when he suffered his first professional loss at the hands of Joseph Parker in 2016.
Manny Robles was in charge of Ruiz last year when he shocked Anthony Joshua to become world champion.
However, Ruiz gained weight for the rematch in December and Joshua reclaimed all his belts with a twelve round unanimous decision.
Ruiz and Robles split shortly after that fight and last week, the 30-year-old announced that he would be working with Eddy Reynoso, who also trains Canelo Alvarez.
"Eddy Reynoso must keep him in the gym, make him work and get rid of the baggage around him (the people that surround him and allow him to do things that he must not do)," Abel Sanchez told Sky Sports.
"Andy must listen. Eddy is a heck of a coach, one of the best in boxing right now."
Sanchez also gave an insight into what it was like to train Ruiz Jr.
"I'm not there to babysit, I am there to coach and develop you if you are willing to listen. You can't develop someone who doesn't want to try anything.
"It was difficult for me because I don't like laziness. We work too hard. I have too many people here working their butts off. To have one person get away with things that nobody else can? That's not proper.
"He doesn't look like a prototypical heavyweight but he has a lot of ability, unbelievable ability.
"He lacks in desire. He doesn't want to do things. He finds an excuse not to do something.
"That's a shame because he could do great things."