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Apple's earnings could be made or broken by iPhone: Pro

Apple is expected to report its fourth quarter earnings on Tuesday. Yet BGC Financials analyst Colin Gillis says a good iPhone sales number this quarter could actually mean bad news for December's report.

"The key number for Apple is going to be the forward guidance for the December quarter, because that is where the [comparisons] become so difficult," Gillis told "Fast Money" traders last week. "And if there is any hint that the iPhone cycle is beginning to slow, it's going to pull the stock lower."

Apple decided to roll out the iPhone 6s/6s Plus in China in September, stumping many analysts tracking the iPhone sales growth. Last year Apple waited until December to make the iPhone 5s/5c available in China making comparisons challenging.

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A larger global mobile slowdown is also in play, according Gillis.

Year over year, global mobile growth has slowed to 8 percent as the larger shift from mobile phones to smartphones edges closer to completion. Gillis says that over time, the smartphone market will begin to mimic the mobile phone market, one of the biggest factors making investors nervous.

Apple iPhone 6s
Getty Images
Apple iPhone 6s

"We see two opposing market dynamics happening, which is that while the broader smartphone market is slowing, Apple is increasing its share of the high end," said Gillis in his most recent note on Apple.

"While we are positive on Apple's attempts to build their own leasing program, and accelerate the upgrade cycle to an annual basis, we suspect that ultimately the smartphone upgrade cycle is going to lengthen," he added.

Gillis is expecting Apple to sell 51 million iPhone units, 12 million iPad units and 4.8 million Macbook units this quarter.