Shares of Darden Restaurants Inc. slumped Wednesday on news about the sudden resignation of Dick Rivera, president of the company's struggling Red Lobster unit.

The stock of the Orlando, Fla.-based casual dining giant dropped $1.88, or 9 percent, to close at $19 Wednesday on heavy volume on the New York Stock Exchange.

The Rivera resignation announced Tuesday has raised questions about Darden's future management.

Rivera, who also resigned his posts as Darden's president and chief operating officer, replaced Edna Morris as head of Red Lobster last September.