Shares of Cablevision Systems Corp fell more than 15 percent in early trading on Friday as investors sold off on uncertainty surrounding the sudden departure of the New York cable operator's well-regarded chief operating officer.

Cablevision announced the sudden resignation of Tom Rutledge, a nine-year veteran of the company, late on Thursday, leading to speculation that he would be joining a larger rival such as Charter Communications Inc . It also raised the possibility that the Dolan-family-controlled Cablevision would be vulnerable as an acquisition target for Time Warner Cable or Comcast Corp .

Analysts at Miller Tabak and ISI Group downgraded Cablevision on the news and others on Wall Street described the news as a major loss for Cablevision.

Although Rutledge renewed a five-year employment contract two years ago that would have taken him through to 2014, Bryan Kraft of Evercore Partners noted that a one-time special cash and stock award totaling $18.5 million he received at the signing of his employment contract became fully vested this month. Kraft said this might be an indication that Rutledge has been planning to resign for some time.

Rutledge's departure was especially of concern to investors because his long-time No. 2 John Bickham stepped down as president of Cablevision's cable operations last month.

Shares fell $2.10 to $11.80 in early trading on the New York Stock Exchange.

(Reporting By Yinka Adegoke; Editing by Gerald E. McCormick, Dave Zimmerman)