The "Incredible Hulk" actor – a former Mr. Universe – is paying secret visits to the "Thriller" star's Los Angeles home to help him prepare for his forthcoming "This Is It" run of shows at London's O2 arena.
A source told Britain's The Sun newspaper: “Lou has been visiting Michael to build him up so he can perform his dance routines.
"But Michael refuses to lift weights. He doesn't want to bulk up."
The 50-year-old singer – who weighs a reported 126lbs and eats just one meal a day - is reportedly battling skin cancer and is said to be terrified gaining weight will damage his chances of beating the disease.
Michael and Lou struck up a friendship two years ago when the "Bad" singer began using the 57-year-old actor's gym.
Lou said at the time: "Michael is more delicate. Our trainer conditioned him for dancing."
"He likes me, he feels safe with me. He was nervous about people looking at him when he's training."
Last month, it was revealed the "Beat It" star would kick off his concert run on July 13, five days later than originally planned.
Shows scheduled to take place on July 10, 12 and 14 have been moved to the end of the residency in March 2010.
