Ever since joining Twitter as chief financial officer in 2014, Anthony Noto has been viewed by Wall Street as a stabilizing presence in an otherwise erratic company. That is leaving some investors uneasy following his unexpected departure to become CEO of student loan provider Social Finance.

The 48-year-old Noto, who ends his stint at Twitter as its chief operating officer, is a familiar face in financial circles. He is a former sell-side analyst and Goldman Sachs investment banker who was involved in Twitter’s ballyhooed initial public offering in 2013.

San Francisco-based Twitter hired a year later to help turn the highly regarded internet startup into a profitable enterprise, and he has earned kudos for the company’s improving performance of late.

“We view this departure as a major loss for Twitter and possibly a negative read into the company’s ongoing efforts to fix the platform and get it back on a growth trajectory,” analysts at SunTrust Robertson Humphrey wrote in a recent client note. “We’ve known Mr. Noto for many years and believe he’s been a strong addition to the Twitter team, bringing not only financial discipline but also credibility with investors at a time of stress for the company.”

Under Noto’s leadership, Twitter slashed operating expenses while investing in growth areas such as video streaming, with high-profile deals with Major League Baseball and the National Hockey League, live events such as the MTV Video Music Awards and a 24/7 video news channel in partnership with Bloomberg.

In other moves aimed at revving up growth, Twitter has also scrapped its 140 character limit and improved the product by using artificial intelligence to show users Tweets that might interest them.

“The end result has been not just more users of the Twitter platform, but a far more engaged user base coming back more often on a daily basis and spending significantly more time on the platform each day,” said BTIG Analyst Rich Greenfield in a recent blog post. “As we have seen over an extremely volatile past 11 months. Twitter’s recovery will not be a smooth straight-line, with much work to be done on the product.”

Although Twitter struggled to ramp up growth following its IPO, Wall Street has looked on it more favorably in recent quarters. The company reported better-than-expected results in October as its monthly user growth topped analyst expectations. The company’s shares have risen more than 40 percent over the last 12 months.