What we do know so far is that Marchionne underwent surgery on his right shoulder several weeks ago and that he had confirmed business appointments for late July. The company has not provided details on Marchionne's illness.
In 2009, U.S. President Barack Obama's administration announced that Fiat would take control of Chrysler LLC, rescuing the American company from bankruptcy.
Elkann originally hired Marchionne in 2003 to serve as a boardmember and then CEO of Fiat, which at the time had just lost $7 billion.
The orchestration of the Fiat-Chrysler merger in 2014 was seen as the height of his achievement, and in recent years he had switched the strategy of the company to focus on SUVs, particularly from the Jeep brand. That ultimate deal eluded him. He could parry questions from the Italian press in one press conference, and deliver sly put-downs of his rivals in English during the next. His office on the fourth floor of Fiat's Turin headquarters was adorned with a black-and-white poster of the word "competition" and a Picasso print bearing the motto, "Every act of creation is first of all an act of destruction".
But it faulted him for having failed to negotiate with "an important sector of Italian workers", which it said created conflict and divisions that are still being felt. Born in Britain and based at the former Chrysler headquarters in suburban Detroit, Manley is expected by bankers and analysts at least initially to follow a five-year plan laid out by Marchionne and other executives in June.
Marchionne resurrected one of Italy's biggest corporate names and revitalized Chrysler, succeeding where the US company's two previous owners - Mercedes parent Daimler and private equity group Carberus - both had failed.