It's no secret that PC sales have been, well, bad for awhile now. Sales of Apple's Mac computers, however, have held fairly well up for the most part.
That may be shifting.
In the third quarter, Mac sales dropped 11 percent to 1.9 million from the period a year earlier, according to an IDC report. What's more, Apple was the only company among the top five U.S. vendors to post a decline in shipments.
"The PC market in general has been in six quarters of consecutive decline, and it's getting cannibalized by tablet sales," BGC Financial's Colin Gillis told CNBC on Thursday.
"You go back and the peak actually happened in September of 2011. PC sales as a total are down 17 percent since then, and if you add up the numbers, it correlates with tablet sales. Tablets are cheaper, tablets are easier and the PC may wind up becoming more of a relegated, niche product."
For Apple, it use to be all about the "halo effect," Gillis said. Consumers would buy the iPhone or iPad, and Apple bet the consumers would get pulled into the entire ecosystem and begin buying MacBook Pros or MacBook Airs. But, that halo effect is fading, Gillis said on "Squawk on the Street."
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"So on the one hand, we have a nice new revenue stream for tablets, and on the other hand we have PC sales slumping and that is a negative," said Gillis, who has a $500 price rating on Apple stock.
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In addition to iPad sales cannibalizing Mac sales, Apple also has to worry about its tablet competition. Amazon, Google even Microsoft are offering tablets priced below Apple's iPad.
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"Price competition is impacting Apple in the tablet space. It's the same pattern as phones. Android tablets have now crossed over the 50 percent line taking market share from Apple," Gillis said.
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Apple's highest margin product is still the iPhone, followed by the iPad and then Macs. Gillis said that he sees only a few catalysts left for Apple, which includes a deal with China Mobile.
(Read more: Apple, China Mobile deal may be coming soon)
"Apple knows its niche," he said. "They keep their prices high. They are trying to keep their margins intact, but unfortunately they are going to lose a segment of the marketplace. It's just, it is what it is."
—By CNBC's Cadie Thompson. Follow her on Twitter @CadieThompson.
