Microsoft CEO Satya Nadella has lost one of his key lieutenants: Qi Lu

Qi Lu, the man in charge of Microsoft's Applications and Services Group, is leaving the company due to health reasons.

His departure was first reported by Re/Code's Kara Swisher and Ina Fried and confirmed by Microsoft to Business Insider. Lu was apparently in an awful bike accident a few months ago and is leaving to focus his energy on recovering.

It was not immediately clear whether Lu's departure is temporary, while he recovers, or a complete exit from the company.

This is a big loss for Microsoft. Lu, who joined Microsoft from Yahoo in 2008, was considered by many inside the company as the quiet, thoughtful counterbalance to Microsoft's outspoken head of Windows, Terry Myerson, sources told us. Myerson has been at Microsoft since 1997.

As we previously reported, early in Nadella's reign, there was a power struggle of sorts going on between Lu and Myerson over who would control Bing and MSN. Lu is known as a diplomatic guy to work for - a peacemaker. Myerson is known as brash and forceful. Myerson won the battle.

Lu's responsibilities included the research and development teams across Microsoft Office, Office 365, SharePoint, Exchange, Yammer, Lync, Skype, Bing, Bing Apps, MSN and the Advertising platforms and business group.

Some of those groups, like MSN, advertising and Yammer, were on the decline and some of them, like Microsoft Office 365 and Skype were on the rise within the company. Lu was also expected to be involved in efforts to integrate LinkedIn's data to various Microsoft apps after that deal closes.

Rajesh Jha, the corporate VP heading up Microsoft's Outlook and Office 365, has been promoted to replace Lu.