Sandoval, 29, showed up to spring training overweight and then lost his starting job to Travis Shaw. Sandoval had played in only three games with six at-bats before being placed on the 15-day disabled list Wednesday with a shoulder issue.
"He needs to be smart enough to say there's a problem," trainer Ethan Banning, owner of Triple Threat Performance in Phoenix, said in an interview with the Boston Herald. "It's like the alcoholic that won't admit he's an alcoholic: well, you can't address that you're an alcoholic if you don't ever admit there's a problem."
Banning worked to get Sandoval in shape prior to his All-Star seasons in 2011 and 2012.
"He's proven to me and shown consistently that he's got to have somebody like me holding his hand," Banning told the newspaper. "And it's not an exercise thing, it's an eating thing. Obviously exercise is an important factor in it, a very important factor, but eating is going to be the component that needs to be managed and monitored. We had a chef on staff that cooked all his meals."
Banning said that when Sandoval went home to Venezuela for Christmas in 2011, the infielder gained 21 pounds in 21 days. When Sandoval returned to work out in Phoenix, Banning did everything possible to keep him out of sight from his team at the time, the San Francisco Giants.
"I would go pick him up at a random location, drive him to the facility so that his car wouldn't be there, so if they dropped in, they wouldn't know he was there," Banning, whose working relationship with Sandoval ended in early 2012, told the Herald. "So for about a three-week period, he had the flu; we had every excuse in the world. We were just trying to rip weight off him again. And it ballooned way out of control."
Sandoval signed a new three-year, $17 million deal in January 2012, went on to have a second straight All-Star season and was named World Series MVP as the Giants won the second of three championships during Sandoval's tenure in San Francisco.
Following the third title in 2014 with the Giants, Sandoval signed a five-year, $95 million deal with Boston.
In his first season with the Red Sox in 2015, Sandoval hit just .245 with 10 home runs and 47 RBIs.
"You need the babysitter," said Banning, who said he has not spoken to Sandoval. "Hey, that's a pretty harsh statement. At the end of the day, I'm speaking truth. ... I love the guy."