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 Group Inc. shares jumped after funds piled into the company, which has been hampered by a federal probe into its business practices and weakening results.

“We think that most of their problems, if not all of them, are temporary in nature,” says Marshfield’s co-portfolio manager Elise Hoffmann.