ShutterstockS&P 500 companies that lost a CEO in 2017 underperformed by 11% over the next 12 or so months, according to a Morgan Stanley report published Monday.Intel employees woke up one morning at the end of June to surprising news: Brian Krzanich, CEO of the company since 2013, had resigned after the board found he had violated company policy by having an affair with an employee. Krzanich's departure came as a surprise to many, including a former board member who told Business Insider's Julie Bort that the decision to find new leadership at the $228 billion chip company was sudden. But Intel quickly replaced Krzanich with an interim CEO in the form of then-CFO Bob Swan, and went on to have a pretty good quarter.Not all companies will be so lucky, according to a new report published by Morgan Stanley on Monday, which found that CEO departures meaningfully impact shareholder value.As a majority of CEO departures are unexpected, this elevated level of turnover suggests that investors should be increasingly aware of companies that face 'key man' risk, or those that have a high level of reliance on a single individual, wrote equity analysts Mark Savino, Jessica Alsford and Victoria Irving.In 2017, CEO departures reached a their highest level since before the financial crisis, according to the report. S&P 500 companies that lost a CEO in 2017 underperformed by 11% over the following 12 months, or since the departure if it was less than 12 months ago.The report identified CEOs across sectors whose departure would have the biggest negative impact on their company's value. We pulled out the seven CEOs in the cloud software-as-a-service (SaaS) space, who investors should take pains to monitor closely.Visit Markets Insider for constantly updated market quotes for individual stocks, ETFs, indices, commodities and currencies traded around the world. Go Now!