Anthony Noto is leaving his role as COO at Twitter (NYSE:TWTR) to join SoFi as CEO.

Noto joined Twitter in 2014 as CFO. When former COO Adam Bain left the company at the end of 2016, Noto was elevated to COO. There was a clear successor to Bain in Noto 15 months ago, but Noto's departure could send Twitter into disarray.

Noto, even before becoming COO, was an integral part of the executive team. He was responsible for Twitter's live video efforts, thanks in part to his connections with the NFL. He also took charge of Twitter's marketing team. As COO, he was in charge of all of Twitter's revenue-generating teams on top of his previous efforts in live video and marketing.

Noto was Twitter's most important executive -- even more so than CEO Jack Dorsey, who splits his time between Twitter and Square (NYSE:SQ). His departure leaves a lot of questions surrounding Twitter.

Twitter's headquarters in San Francisco

Image source: Twitter, Copyright Aaron Durand (@everydaydude) for Twitter, Inc.

Who's running things?

With Dorsey's attention split, Noto was largely responsible for picking up the slack at Twitter. He's the guy that handled most analysts' questions on earnings calls and drove the revenue strategy for Twitter. (Dorsey is largely focused on the product.) Twitters says Noto's roll will be filled by other Twitter employees following Noto's departure.

For Twitter, Noto's departure comes as the company is in the midst of a crucial turnaround. The company is expected to report profitable earnings on a GAAP basis for the first time when it releases its fourth-quarter results next month. Moreover, Noto has been crafting a new message to take to marketers that focuses on Twitter's improving user engagement, as daily user growth has dramatically outpaced its monthly user growth over the past year and a half.

Last year, Noto shut down underperforming ad products to focus on a few select products. Combined with competitive pressure, Twitter saw declining ad revenue in each of the first three quarters for the year. 2018 is a key time in Twitter's turnaround strategy as it looks to return to ad revenue growth. Without a full-time CEO or any COO, it's not clear the company can manage the situation properly to complete the turnaround.

Just when things were starting to look better

Twitter underwent a lot of turnover in 2016, with several executives jumping ship as the company struggled to attract new advertisers and grow its audience. That year, Twitter lost its COO, CTO, VP of product, VP of engineering, VP of global media, and more than a handful of other execs. It also made the decision to shutter six-second video app Vine that year.

2017 featured much less turmoil, and management, particularly Noto, did a good job of handling investors' expectations. Noto's departure could set the stage for a much more tumultuous 2018.

Speculation is already circulating that Twitter may look to sell itself once again. It tried to sell itself at the end of 2016, but it was unable to find a buyer. Walt Disney (NYSE:DIS) was among the companies that expressed interest. A media company could be a good fit for Twitter, particularly one focused on news -- Twitter's strength.

But if Twitter couldn't get a deal done back in 2016, there's little reason to expect one this year. Disney is almost certainly out after announcing its plans to acquire most of Twenty-First Century Fox's (NASDAQ:FOXA) assets. Perhaps what's left of Fox may be interested in Twitter.

Buyout speculation could continue to support Twitter's stock price for a long time, which makes it particularly risky for investors. And that's on top of the risk that Twitter won't be able to execute on its stand-alone plans without Noto taking the lead.

This article represents the opinion of the writer, who may disagree with the “official” recommendation position of a Motley Fool premium advisory service. We’re motley! Questioning an investing thesis -- even one of our own -- helps us all think critically about investing and make decisions that help us become smarter, happier, and richer.