Sergio Marchionne, the chain-smoking, deal-making boss of Fiat Chrysler Automobiles, was an unlikely auto executive, spending much of his career in other industries. But in the last 14 years, he helped bring back two carmakers from the brink of collapse.

On Saturday, Mr. Marchionne, 66, was abruptly replaced. Fiat Chrysler revealed that he had become gravely ill since having shoulder surgery on July 5. The company named Mike Manley, 54, who is the head of the automaker’s North America operations and its Jeep and Ram truck brands, as Mr. Marchionne’s successor, effective immediately.

Fiat Chrysler declined to provide details about Mr. Marchionne’s health, but it made clear his condition had worsened significantly and suddenly.

“I am profoundly saddened to learn of Sergio’s state of health,” John Elkann, the great-great-grandson of Giovanni Agnelli, the Fiat founder, and the chairman of the family’s investment company, Exor, a major Fiat Chrysler shareholder, said in a statement. “It is a situation that was unthinkable until a few hours ago, and one that leaves us all with a real sense of injustice. My first thoughts go to Sergio and his family.”