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. He is to remain as a consultant with the company.

Darden said chairman and chief executive Joe Lee will assume operating responsibilities for both Darden and Red Lobster until a new Red Lobster president is named by the end of May.

Compounding investors' concern was Darden's latest monthly sales report on Tuesday. It showed that while its Olive Garden Italian restaurant business continued to post gains, Red Lobster's sales were off 12-13 percent for December on a same-store basis that generally refers to stores open at least a year. The drop reflected a double-digit decline in guest traffic.

Also, several brokerages cut their ratings on Darden's stock, along with earnings estimates for this year and fiscal 2005. Downgrades came from Credit Suisse First Boston, J.P. Morgan Securities, A.G. Edwards & Sons, McDonald Investments and Oppenheimer.

Efforts to reach Rivera for an explanation of why he left Darden after six years in its executive suite weren't immediately successful. Darden's announcement of his resignation quoted him as saying he sought ``to look at some exciting startup opportunities as well as smaller companies with growth potential'' in the restaurant industry.

Beyond a successor at Red Lobster, Rivera's departure brings into focus the issue of who eventually will succeed Lee, who is 62 years old.

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Darden's stock prices tumble on news of Rivera quitting