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.

Shares of some of UnitedHealth’s peers got a bump in the wake of Berkshire’s investment in the healthcare giant, and they all look cheaper based on price relative to expected earnings.