Activist investor Brigade Capital Management opposed the deal by Humana Inc. and two private equity firms to buy Kindred Healthcare Inc., calling the deal value "disappointing and grossly inadequate."

Humana, Welsh Carson Anderson & Stowe and TPG Capital Management LP recently agreed to buy Kindred for $4.1 billion, or $9 per share.

The investor, which holds a 5.8% stake in the medical-facility operator, plans to vote against the transaction.

In a letter, Brigade said the "deal price is not reflective of Kindred's intrinsic value and will short-change existing shareholders," adding that from the perspective of maximizing shareholder value, this is a "terrible time" to sell the company.

The investor also said the offer "severely undervalues" the company and "ensures that the buyers — rather than existing shareholders — will reap the benefits of the value enhancement the improved business will generate."