Apple's (AAPL - Get Report)  ambitious goals for its growing Services business may take longer than expected to achieve, according to Mizuho analyst Abhey Lamba. 

As the company's mainstay iPhone segment faces potential headwinds, CEO Tim Cook has tried to turn Wall Street's focus to the company's Services segment, saying the unit is "well on its way" to becoming the size of a Fortune 100 business. On several occasions, Cook has reiterated his belief that Services, which includes the App Store, Apple Music, iCloud, iTunes and Apple Pay, can double its revenue by 2020.  

Services is already Apple's second largest business, with total sales of about $24.3 billion in 2016, compared to the iPhone's $136.7 billion that same year. The unit has seen double-digit percentage revenue growth for the past eight quarters, according to FactSet.

In a note to clients this week, Lamba argued that "muted growth" in iPhone sales and other products might slow down the pace of revenue growth in Services. Apple believes, however, that its Services unit can increase the amount of money it makes from its existing user base. 

"Overall, although services likely offer better relative profitability (versus other categories) and have the potential to become a bigger part of the revenue mix over time, we think it is too early to give the company credit for its goals, especially given the varying profitability mix among the different services in the portfolio," said Lamba, who downgraded Apple shares to Neutral from Buy on Monday.

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Apple doesn't give sales metrics for each of its individual Services units, but Lamba believes the App Store and Apple Music have been the most profitable. Apple's iCloud business isn't generating as much money, he added. 

On top of that, Lamba noted that users outside the U.S. may be spending less money within Apple's Services ecosystem. The average Services spending per customer worldwide was $100 per year in fiscal 2011, but that figure fell to about $50 in the beginning of 2017. The slowdown is likely tied to Apple penetrating geographies outside the U.S., U.K. and other developed countries, where "services attached per iPhone is lower," Lamba said. 

Wall Street expects Apple's services revenue to rise to approximately $65 per user by 2019, but Lamba believes the consensus estimate may be too aggressive. 

"While we agree that the metric can grow from the current level due to Apple's strong focus in the segment, we think the amount of growth could be more muted as incremental expansion of users is happening in regions where attach rate of services is low," Lamba explained. "At the very least, we think there is limited upside to revenue estimates from the segment."

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Editor's Pick: Originally published Jun. 12.