iPhone 6s. Photo: Apple.

Apple (NASDAQ:AAPL) will turn in its earnings report for the September quarter next week. When it does, shares could sell off if a recent note proves accurate.

Earlier this month, analysts at Pacific Crest warned that Apple's iPhone 6s was suffering from disappointing sales. Although initial demand was strong, Pacific Crest believes sales have declined meaningfully in recent weeks.

Sixty-seven million iPhone sales in the calendar fourth quarter
The iPhone 6s was only available for a few days during the quarter, as it made its debut on Sept. 25. If sales have slowed down in recent days, it won't be reflected directly in the results Apple reports next week.

But it could weigh on Apple's guidance for the crucial December quarter. If Apple's management doesn't comment on the recent trends it's seeing with the iPhone 6s, analysts are likely to ask during its quarterly earnings call. The iPhone continues to generate the bulk of Apple's revenue and earnings, and any hints of tepid demand could take a toll on shares.

Pacific Crest expects Apple to sell 67 million iPhones in the December quarter. Other analysts are more optimistic -- the consensus is around 76 million. Last year, Apple sold 74.4 million iPhones in the same time period, up from 51 million the year before. Last year's figure will be hard to top, and analysts are projecting only modest growth; but if Pacific Crest is right, Apple's iPhone business could contract.

The iPhone 6 and iPhone 6 Plus were the first iPhones to offer larger displays, a long-desired feature. This year's iPhone 6s and iPhone 6s Plus are objectively better devices, but they aren't as revolutionary. Consumers who purchased the iPhone 6 last year may be content with their current devices, while the number of Android converts could have stabilized.

At the same time, new carrier programs have introduced a level of uncertainty. Phone financing plans allow consumers to upgrade more often, but they also reward those who hold on to their older handsets for a longer period of time. By separating the cost of the device from the cost of the service, consumers who choose to pay off their handset and continue to use it can see their monthly bill decline by upwards of 30%. How this will affect upgrade cycles and iPhone sales remains unknown.

Pacific Crest cited conversations with supply chain sources to explain its iPhone call, as well as surveys of carrier stores. Last year, many stores were sold out the iPhone 6 in October -- this year, few stores are sold out of the iPhone 6s. At the same time, Apple appears to have cut its component orders.

Pacific Crest has been wrong before
Those are obviously subjective measures, and Pacific Crest's assessment could prove overly bearish. There's reason to doubt its outlook.

In September, before Apple announced iPhone 6s sales for its opening weekend, the company put out a vague statement declaring that iPhone 6s pre-orders were "exceptionally strong." Pacific Crest disputed that notion, arguing that early demand for the iPhone 6s appeared to be meaningfully weaker than its predecessor. At the time, analysts cited search trends, surveys, and shipment times.

Later, Apple officially announced that it had sold 13 million of the iPhone 6s in its opening weekend, setting a new record, and smashing last year's total by 30%. Clearly, Pacific Crest's assessment was wrong.

Apple investors should hope for a similar outcome in the weeks ahead.

This article represents the opinion of the writer, who may disagree with the “official” recommendation position of a Motley Fool premium advisory service. We’re motley! Questioning an investing thesis -- even one of our own -- helps us all think critically about investing and make decisions that help us become smarter, happier, and richer.