Selling 5 million smartphones in three days is no easy feat, and with so many people rushing to buy Apple’s (AAPL) new iPhone 5, the sustainability of the remarkably heavy iPhone demand we have seen thus far is being called into question. In a recent note to investors, RBC Capital Markets analyst Amit Daryanani wrote that he believes iPhone sales will be even hotter than initially expected in the September quarter and he increased his sales estimates to 24.8 million units from his earlier projection of 20 million iPhones. In the December quarter this year, however, Daryanani slashed his estimates by nearly 15%.

The analyst had previously seen Apple selling 57 million iPhones in what many industry watchers expect to be Apple’s most profitable quarter of all time.But Daryanani has now cut his estimate to 49 million units, claiming a bigger-than-expected September quarter is partially responsible for the lowered projections. His new fiscal fourth-quarter estimates only account for just over half of the discrepancy though, and Daryanani’s new numbers are still 3.2 million units lighter than his earlier estimates over the six-month period ending in December.

Daryanani speculates that continued supply constraints may hinder iPhone sales, but he notes that the iPad mini could provide a significant boost in Apple’s fiscal first quarter. If the rumored device does launch in the coming weeks as expected, the analyst sees Apple selling 8 million units in the December quarter, which would bring in about $2 billion in revenue.

RBC maintained its Outperform rating on Apple stock with a 12-month price target of $750.

Zach Epstein has worked in and around ICT for more than 15 years, first in marketing and business development with two private telcos, then as a writer and editor covering business news, consumer electronics and telecommunications. Zach’s work has been quoted by countless top news publications in the US and around the world. He was also recently named one of the world's top-10 “power mobile influencers” by Forbes, as well as one of Inc. Magazine's top-30 Internet of Things experts.