The "Let's Go Crazy" icon, 57, was found dead at his Paisley Park home in Minnesota on Thursday.
Magnoli posted a statement on his website calling Prince not only his creative partner, but a friend.
"We shared tremendous times together during and after the making of 'Purple Rain'. He was a good man, complicated and diligent, and like men of his caliber, he always demanded from himself and others the very best," he wrote.
"Purple Rain" grossed more than USD 80 million at the box office in 1984 and became a cult classic. The film won an Oscar for best original song score.
While the cause of Prince's death is still unknown, officials from the Midwest Medical Examiner's office confirmed they have received his body and an autopsy will be performed today.