Ming-Chi Kuo of KGI Securities said in the note to investors he now estimates Apple will be able to produce just 40 million handsets before the end of the year, down from an earlier estimate of 50 million.
The backup, Kuo estimates, will result in 2018 iPhone X shipments in the 80 million to 90 million range.
Apple has previously told CNBC it had built the shipping dates into its forecasts.
Still, analysts anticipate delays stemming from supposed issues with the OLED edge-to-edge screen and the complex new Face ID facial recognition system.
The extent of the delays have been previously unknown, giving some analysts, and likely potential buyers, pause.
"If you have consumers that place orders in early November, and they can't get a phone until 2018, then I think it matters," Toni Sacconaghi told CNBC's "Fast Money: Halftime Report" earlier this week. "There are other attractive offerings out there. And while ecosystem loyalty is high, if you were hoping to get this great new phone in September, and now you're waiting until 2018, I think you'll get .... switchers."
Sacconaghi, senior technology research analyst at Bernstein, said the backups could ultimately push buyers to rival phone-makers like Samsung.
Preorders for the coveted 10th anniversary iPhone model start Oct. 27, more than a month after preorders for the iPhone 8 and iPhone 8 Plus, which began Friday. The iPhone X is expected to start shipping on Nov. 3.
— CNBC's Anita Balakrishnan contributed to this report.