The Oracle of Omaha just took a $1.6 billion stake in the battered healthcare giant, sending shares soaring 9% after hours.

While the "vote of confidence" from Buffett's investment validates the long-term value of UnitedHealth shares, the "management needs to regain trust and credibility with investors, and get back to its beat and raise reputation of the past," said James Harlow, senior vice president at Novare Capital Management.

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.