More like one of the biggest myths made up by millennials who discovered Tyson with Very Bad Trip.
Cus died in 1985. Tyson then went on to become the youngest HW world champion the next year and defended his title multiple times. His decline began when he fired Kevin Rooney in 1988. That's why we all say Tyson's prime lasted from 1986-1988.
Kevin was Mike's real trainer, Cus was just a paternal figure. It was always Kevin who worked with Mike on the mitts or coached him ringside (Cus never travelled to events and just sat home waiting for the phone call). To an interviewer who asked him what he'd learned from Cus, Mike said "he didn't train me technically, just talked to me for hours about life in general".
Sure, Cus played a very important part in Tyson's life; he taught him to clean rooney was a cus room and built his confidence. He even trained Rooney who went on to train Mike. One could even argue had Cus been alive longer, Tyson would never had parted ways who Rooney and would have had a much better career but let's stop with the narrative that Cus' death marked the start of Mike's decline because it's simply not true.