Buffett owned UnitedHealth shares before selling his stake in 2010, though his Berkshire Hathaway recently acquired shares.

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 Inc. took new positions in UnitedHealthcare Group Inc. and homebuilder D. R. Horton Inc., and went deeper into other homebuilding and home-related companies, according to a filing late Thursday.