Microsoft’s plan, announced Friday, to replace Steven A. Ballmer as its chief executive does not exactly follow — at least to people outside the company — the way they draft these things in business school.

But Mr. Ballmer and Microsoft’s board have been considering the possibility of his retirement for some time. Still, because of Mr. Ballmer’s larger-than-life personality, the board’s reluctance to push back and the company’s recent product and financial problems, finding a new chief executive for Microsoft was never going to resemble a cut-and-dry, business-school case study, according to people with knowledge of the company.

“No one is an obvious candidate,” said Michael A. Cusumano, a professor of business and engineering at the Massachusetts Institute of Technology who studies strategy in the computer software industry. “All the really interesting people who were in the company over the last dozen years who might have been have left. I also find it hard to imagine they could bring an outsider in. Microsoft is known for having quite a lot of powerful groups within the company and they make life very difficult for anyone who tries to oversee them.”

Succession planning is a delicate issue for many companies, particularly one like Microsoft, where Mr. Ballmer has been a senior employee since 1980 and chief executive since 2000, and his longtime friend, Bill Gates, Microsoft’s co-founder, remains chairman.