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.

Warren Buffett’s Berkshire Hathaway sold shares in Apple during the second quarter, further paring its position in the iPhone maker, while adding UnitedHealth Group to its portfolio.The Omaha, Neb., company disclosed its stockholdings in a regulatory filing on Thursday.