Last week, the mobile network AT&T reported subdued demand for upgrades. UBS expects iPhone sales to be around 45 million in the quarter, flat on last year, although it believes revenues and profits will grow, helped by rising iPad sales and demand for more expensive plus-sized iPhones.
Analysts will grill Tim Cook, Apple's chief executive, on whether slow iPhone 8 sales are a sign that shoppers are waiting for the more expensive iPhone X model or merely buying fewer handsets.
"The weakness in the iPhone 8 probably is more due to strong demand for the X than a disappointing cycle," UBS said.
The iPhone X sold out almost immediately when pre-orders opened, with some shoppers told to wait several weeks for the phone to be delivered. It suggests that even if there is demand for the high-end device, Apple is struggling to make enough of them.
The iPhone X's new design and complicated 3D sensor system have led to widely-reported production hiccups. It is going on sale two months after its release and supply is unlikely to catch up with demand for months.
Last week Apple was forced to deny a report that it had lowered production standards to boost supply.
Apple's last experiment with releasing two different versions of the iPhone was in 2013, when it unveiled the 5s and a cheaper iPhone 5c.
The latter sold in disappointing quantities and Cook admitted the company had overestimated demand for it.
Apple shares are trading at close to all-time highs, suggesting investors remain confident about the company's future. Its guidance for the current quarter, typically Apple's most profitable period of the year, will also be eagerly awaited this week.
For the next quarter, it is expected to report record iPhone sales and profits.