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.

Berkshire Hathaway made a surprising $1.6 billion investment in UnitedHealth Group during the second quarter, even as the insurer faced challenges.