Andros Georgiou said the singer had resumed taking “hard drugs” toward the end of his life, but denied his death was suicide.
The 53-year-old was found dead at his home in Goring-on-Thames, Oxfordshire, on Christmas Day.
Police said a post-mortem had proved “inconclusive” and the results of further tests are yet to be revealed.
His death is being treated as “unexplained but non-suspicious”.
Speaking to the BBC’s Victoria Derbyshire programme, Mr Georgiou said hard drugs “had been back in [Michael’s] life” and crack cocaine was a “favourite”, but he denied the singer used heroin.
“I just think he took too much of something, mixed with antidepressants and other drugs he was on - with alcohol,” he said.
“I think his heart just stopped beating.”
Mr Georgiou said Michael had taken steps to overcome his use of illegal drugs.
However, after speaking to people who knew the singer towards the end of his life, he believed he had been “dragged back into the dark side”.
Detectives from Thames Valley Police have questioned the last people to see Michael alive as part of their investigation.
Mr Georgiou said he wanted to “get to the truth of what happened” and to know what the singer may have taken and how he could have acquired it.
The former record producer rejected speculation that Michael had taken his own life after suffering from depression.
He said: “I believe he had suicidal thoughts, because his mental health was all over the place. But I don’t believe this was suicide.”
Mr Georgiou worked with Michael at the height of his fame, although the pair became estranged in 1998.
He paid tribute to the “incredibly generous” star who was “one of the nicest people you could ever meet”.