Colin Sebastian, an analyst at Robert W. Baird, predicted that YouTube would generate $15 billion in advertising revenue in 2018.

Sebastian said Alphabet, YouTube's parent company, was likely to start breaking out YouTube's finances.

Amazon became a lot more appealing to investors when it broke out finances for Amazon Web Services.

Alphabet, the umbrella company for Google and YouTube, has never revealed how much money it makes from YouTube — which seems crazy, as it's probably at least as valuable as Netflix.

It also doesn't formally disclose how much it makes from its cloud-services arm, Google Cloud.

Colin Sebastian, an analyst at Robert W. Baird, thinks that keeping the financials for these two fast-growing businesses quiet means Alphabet is undervalued as a company.

In a session with reporters on Thursday, he predicted YouTube would pull in $15 billion (£10.7 billion) in advertising revenue this year. For comparison, Netflix's revenue last year was $11.6 billion.

Sebastian said that Alphabet's lack of transparency with YouTube and Google Cloud might be hurting its overall valuation but that it isn't likely to stay mum for much longer, at least for YouTube.

"Our view is that Google will provide a YouTube number, if not this year, then likely next year," he said.

To understand why, he pointed to something Amazon did in 2015, saying Google could be looking for its Amazon Web Services moment.

For a long time, Amazon looked as if it had no profits

We know that Amazon hasn't historically profited much from its main business of selling stuff to people.

But the firm has started to make more money in other businesses. One example is Amazon Web Services, the cloud-infrastructure technology that underpins not only Amazon but much of the internet.

It's a high-margin business and a huge profit driver for Amazon, propping up the rest of the company for several consecutive quarters. This is a novelty for investors, who are used to Amazon's thin retail margins and low profitability.

Though it appeared to be making a lot of money from AWS, Amazon for years chose not to disclose its cloud revenue. Sebastian was among the prominent analysts suggesting in 2013 that Amazon had a cloud gorilla on its hands, estimating the business was making $1.5 billion a year.

When Amazon finally decided to disclose AWS revenue in 2015, the stock popped.

Google might be looking for a similar pop, Sebastian said (emphasis added):

"If you recall when Amazon disclosed its AWS numbers, the stock ripped in part because you could actually assign a value to that business. Our view is that Google will provide a YouTube number, if not this year, then likely next year.

"Maybe they will be required to by disclosure rules — but they may want that AWS moment in which the market can assign a ... multiple to YouTube. YouTube is probably a $15 billion business, and yet they don't disclose that number."

To be clear, Sebastian wasn't comparing YouTube to AWS in terms of value, but saying Wall Street forgets Alphabet is sitting on a video giant.

Google Cloud is another monster business in the works

While Google doesn't break out how much it earns from Google Cloud, its CEO, Sundar Pichai, has told investors it brings in $1 billion quarterly, suggesting $4 billion in annual revenue.

For Sebastian, that's another reason to be positive about Alphabet's valuation.

Google Cloud — a major rival for AWS, the jewel in Amazon's crown — is likely to experience similar growth, according to Sebastian. That's good news for Alphabet bulls, especially if Google starts breaking out cloud revenue.

"Will it take Google the 12 years it took Amazon?" Sebastian said. "No, Google will probably have a $20 billion cloud business in five years. And from an Amazon perspective, probably $150 billion of its market cap is probably AWS. [Cloud] is not recognised within Alphabet's valuation — I think transparency will go a long way."