Steve Jobs, Apple Inc.'s (NASDAQ:AAPL) star CEO, announced today he is taking a medical leave.

At my request, the board of directors has granted me a medical leave of absence so I can focus on my health... I love Apple so much and hope to be back as soon as I can, wrote Jobs in an email to Apple employees.

On this news, European-traded Apple stocks plunged about 7 to 8 percent (U.S. exchanges are closed for Martin Luther King day).

RBC Capital analyst Mike Abramsky's note captured the market's concerns:

In our initiation we commented regarding Steve Jobs as being Apple's biggest asset -- and its biggest risk. Upon his return he presided over the launch of iPhone 4, the iPad, the refresh of the Mac Air and other innovations, he wrote, according to Fortune.

However, hedge fund manager David Einhorn -- an Apple investor -- previously said one shouldn't worry too much about Jobs' health.

Einhorn, of course, is a legendary stock-picking hedge fund manager whose advice should not be regarded lightly.

For a premium company like Apple, investors usually pay 40-times, 60-times, or even 80-times earnings, he said on Bloomberg TV.

Apple, however, is only trading at 21-times its earnings. When one subtracts the billions of cash it holds on the balance sheet, the company actually trades at the broad market's multiple, which is around 14-times the earnings.

Einhorn made the comments on December 16th, when Apple shares were trading at $321. In the trading session before Jobs' medical leave announcement, Apple shares closed at $348.

During the December interview, a Bloomberg anchor asked Einhorn about the risk of the company launching an unsuccessful product.

Apple is not dependent on a single product, responded the hedge fund manager.

The iPad will likely be successful and it's in the very early stages of its product life. Furthermore, the iPhone will be available on Verizon soon, he said.

There is a lot of obvious, easy growth that is still ahead of the company, he said.

Do you think about Steve Jobs' health? Isn't there a premium to Apple's stock because of him? asked the anchors.

With the stock trading, net of the cash, around the market multiple, this isn't the sort of thing that one needs to worry unduly about, responded Einhorn.

However, he did later warn that if something unfortunate does happen, one needs to reassess one's investment at that point.