''What makes him very hard to replace is his charisma,'' said industry analyst Rob Enderle.
Jobs sent an e-mail message from his hospital bed Sunday to Apple and Pixar employees announcing he underwent successful surgery to treat a form of pancreatic cancer. The cancer is extremely rare and easily cured if diagnosed early, as Jobs says it was in his case.
Jobs' e-mail said he does not have a deadlier and more common form of pancreatic cancer, called adenocarcinoma.
Apple shares declined Monday, slipping 2.35 percent, or 76 cents, to close at $31.58 on the Nasdaq Stock Market. Pixar shares fell 42 cents, or less than 1 percent, to close at $67.82.
Jobs, 49, assured employees and investors he expects a full recovery and plans to return to work next month.
Timothy Cook, Apple's executive vice president of worldwide sales and operations, will lead Apple. Pixar president Ed Catmull will lead Pixar. Catmull and creative head John Lasseter already handle most Pixar operations. Jobs and friend Steve Wozniak founded Apple Computer in 1976, five years before IBM Corp. jumped into the personal computer market.
In 1984, the company released the Macintosh, the first commercially successful computer to have a graphical user interface that mimicked a physical desktop. It was eventually copied by IBM-clone computers, which became more dominant.
Jobs left Apple in 1985 following a struggle with the board, but made a triumphant return in 1997 when Cupertino-based Apple was struggling. He is widely credited for Apple's renaissance with a string of innovative products - the iMac, PowerMac and PowerBook computers and the popular iPod portable music player.
