SAN FRANCISCO — Yahoo CEO Marissa Mayer is losing a precious security blanket now that she is spinning off the Internet company’s prized stake in China’s Alibaba Group.

The breakup announced Tuesday will transfer ownership of 384 million shares of Alibaba stock, currently worth $39 billion, into a new entity called SpinCo. Those holdings, part of an astute investment made nearly a decade ago, represent the main reason that Yahoo’s stock has more than tripled since Mayer became CEO two-and-half years ago.

Investors viewed Yahoo as another way to own a piece of Alibaba, a rapidly growing e-commerce company that is expected to become even more successful during the next decade as more of China’s population gains online access.

As long as there was a link to Alibaba, it almost didn’t matter that Yahoo’s own digital services have been struggling to generate more revenue during the past six years, even as advertisers have been pouring more money into digital marketing.

With the Alibaba stake jettisoned by the end of the year, Mayer will be under greater pressure to prove she can rejuvenate one of the Internet’s oldest and best-known companies.

“The heat is on,” Outsell Inc. analyst Randy Giusto said. “This (spinoff) exposes the fact that Yahoo still is not growing in key areas where it has been investing.”