By Douglas A. McIntyre, 24/7 Wall St.

When a company's stock drops 35% in six months, its CEO should probably be rethinking his future. That even applies to Tim Cook, Steve Jobs' hand-picked successor at Apple (AAPL). 

Apple's board, like any other responsible one, has an obligation to find the best person possible to run the company. The Apple board many consider Cook a liability if new products fail in the marketplace, the company misses a number of its financial forecasts and its share price continues to fall rapidly. 

Cook may have stumbled through enough tests that his ouster could be just months away.

Who would replace Cook? The list is fairly short, given Apple's size and the complexity of its products and technology.

Apple would not turn inside the company to pick a new chief executive. Too many of Apple's most senior management have been with there too long and are tainted with their association with Apple's troubles.

Potential new CEOs for Apple include:

John Chambers of Cisco Systems (CSCO) is considered the dean of Silicon Valley CEOs. Chambers, who is almost 65, has captained Cisco since 1995. The company's revenue over that period has grown from less than $2 billion a year to nearly $50 billion. Cisco's operations are vast and among the most complex in the tech industry.

A dark horse for the Apple job is the chairman of arch rival Google (GOOG) -- Eric Schmidt. He is no longer CEO of Google, having been moved out of that job by the board and replaced by Larry Page. Google's shares have reached an all-time high, and much of the credit for the company's success should go to Schmidt. But he may want a second act after being pushed upstairs at Google. If he joined Apple and turned it around, he could be considered the greatest CEO in tech history. That, by itself, could be an enticement. And Schmidt has another advantage. He is a former Apple board member.

Retired International Business Machines (IBM) CEO Sam Palmisano led one of the world's largest technology companies through a transformation that helped it diversity into software and services and move further away from hardware. He ran IBM during the mammoth integration of PricewaterhouseCoope​rs Consulting in 2002, considered one of the signature moves in reinventing IBM.

The most risky move Apple could take is to hire former Hewlett-Packard (HPQ) CEO Mark Hurd, who was pushed out of the company and is now co-president of Oracle (ORCL). Oracle founder Larry Ellison said that Hurd was one of the tech industry's most skilled leaders and recruited him within weeks of his ouster from HP. Hurd is known as a hard-nosed operator who ran one of the most diverse tech companies in the world, and ran it well.

There is a slight chance Apple would turn to its own board. Chairman Arthur D. Levinson built biotech company Genentech. He was made CEO of that company in 1995 and is credited with making it into a global giant. Levinson is a prominent scientist and has been cited on several occasions as one of the best CEOs in America.

Cook may not make it until the end of 2013. Apple needs a new CEO short list.

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