Shares of UnitedHealth Group surged more than 13% on Friday after billionaire Warren Buffett's Berkshire Hathaway bought 5 million shares of the company, providing a shot in the arm for investors who think the health conglomerate will turn around under its new CEO.

As UnitedHealth Group wrangles with a federal investigation, an executive shuffle and a tanking stock price, a bullish ally has emerged: Warren Buffett. The Eden Prairie-based health giant’s stock price jumped more than 10% before trading opened Friday, following news that Buffet’s Berkshire Hathaway acquired about 5 million shares.

UnitedHealth shares soared after Warren Buffett revealed a stake of 5 million shares, worth about $1.6 billion, in the troubled health insurer. The "Big Short" investor Michael Burry and Appaloosa Management's David Tepper also disclosed sizable stakes in the company.