Last year, sales of the iPhone 6 in China were a nice bonus. This year they're a must-have if Apple is going to continue to show growth for the iPhone 6S, which is facing some tough comparisons to its predecessor.

Apple on Monday announced that sales of the iPhone 6S are on pace to beat the iPhone 6's first-weekend records. But that comes with a big caveat in the form of China, which accounted for a significant percentage of orders, according to Piper Jaffray analyst Gene Munster.

Last year, initial sales numbers for the iPhone 6 didn't include China because the government had not yet approved the device for sale. This year, Munster anticipates China will account for 2 million of the expected 12 million to 13 million iPhone 6 and 6S Plus smartphones in the first weekend. Without China, sales Apple would see little growth over the 10 million phones it sold in the first weekend of iPhone 6 sales last year.

Apple did not disclose exactly how many of its latest smartphones were sold in the first 24 hours of preorders. Last year, Apple said the iPhone 6 set a 24-hour preorder record of 4 million units, up from 2 million after the launch of the iPhone 5 in 2012. The iPhone 5S had no preorder period.

Aside from China, New Zealand was the only other new country added to the mix of iPhone 6S launch countries. Overall, Munster expects marginal iPhone sales growth during the 6S cycle -- up 3 percent to a projected 238.5 million iPhones sold next year, compared to a projected total of 232.2 million iPhones sold during the iPhone 6 cycle in Apple's 2015 fiscal year.

Despite the economic downturn in China, Apple remains positive about the smartphone market there. “I get updates on our performance in China every day, including this morning, and I can tell you that we have continued to experience strong growth for our business in China through July and August,” wrote Apple CEO Tim Cook in an email to CNBC in August. “Growth in iPhone activations has actually accelerated over the past few weeks, and we have had the best performance of the year for the App Store in China during the last two weeks.”

Following the preorder period, shipments of the iPhone 6S and in-store sales will begin on Sept. 25 in Australia, Canada, China, France, Germany, Hong Kong, Japan, New Zealand, Puerto Rico, Singapore, the U.K. and the U.S.