UnitedHealth Group emerged as a health care colossus over the past decade and a half, earning one of the highest stock market values in the nation. But in the last two years, it has been hit with just about every misfortune that can befall a company:

A gargantuan cyberattack. Federal investigations, including a criminal inquiry into one of its most important businesses. The killing of a top executive. A public relations crisis. Disappointing profits. A plummeting stock price.

In May, facing a collapse of confidence among investors and the public at large, its chief executive, Andrew Witty, stepped aside, and the company abruptly summoned its former C.E.O., Stephen Hemsley. Mr. Hemsley, 73, agreed to serve a second stint for at least three years in exchange for $60 million in stock options.

He is betting on the same strategy he employed when he last ran the company in 2017: UnitedHealth, with $400 billion in revenue, can use its size and scale to wring steady profits from its vast portfolio of businesses, which include the nation’s largest health insurer. He promised an intensive review of the company’s operations, and he will report its latest financial results on Tuesday.

Mr. Hemsley may be able to reassure investors by offering them more clarity about UnitedHealth’s profits and what steps it has taken to improve them.

But it is unclear what the company can do to repair its tarnished image and its financial prospects over the long haul. Regulators have resisted allowing the conglomerate to make major acquisitions because it is already so big. And the American public is deeply skeptical that it can deliver better, cheaper care rather than milking the system for profits.

As care has shifted from different locations, moving from hospitals to doctors’ offices to nurses caring for patients in their homes, UnitedHealth is “just following that and trying to integrate, coordinate it and make money off it,” said James C. Robinson, a health policy and management professor at the University of California, Berkeley.