Apple slumped after Jobs was forced out of the company in the 1980s. Since he returned and launched Apple's revival in 1997, no other technology CEO is seen as so completely coupled to his firm's success. It is "hard to imagine a replacement for Jobs' vision, his unparalleled and charismatic marketing leadership and his ability to drive the company to exacting standards and products," Sanford Bernstein analyst Toni Sacconaghi said in a research note Tuesday.

He called Apple's level of disclosure about Jobs' health problems "inadequate," but didn't ask any further questions about Apple's CEO on Tuesday's call.

Jobs has been in poor health since he was diagnosed with a rare form of pancreatic cancer in 2004. This is his third medical leave since he took a month off to recover from pancreatic surgery nearly seven years ago. He also took nearly five months off in early 2009, during which time he underwent a liver transplant, and -- -- treated his cancer with a special type of "hormone-delivered radiotherapy" at Switzerland's University Hospital of Basel.

When he left in 2009 Jobs said when he expected to be back at work, but this time he didn't say when he expects to be back. The idea of an Apple without Jobs has spooked some investors, who sold off Apple's stock after the news was made public. By the end of trading in the U.S. Tuesday, Apple's stock [AAPL] had dropped nearly 8 percent, to $340.65.

however. Revenue grew by 71 percent year over year during the last three months of the year, said the other executive on Tuesday's call, Chief Financial Officer Peter Oppenheimer. "We sold more Macs, iPhones and iPads than in any previous quarter in Apple's history," he said.