Yagin's manager, Jason House, confirmed the news to MMA Fighting's Ariel Helwani, which was originally reported by MMAWeekly.com.
According to House, Yagin, a Honolulu native who formerly trained with San Diego's Alliance MMA, became woozy after returning home from a sparring session, and checked himself into a local hospital.
"He suffered it in sparring on Saturday," House said. "The injury was not the result of a certain strike or exchange. [He] came home, had a headache, started to vomit whatever liquids he drank and then decided to go to the ER the next day."
House said that according to Yagin's family, doctors said Yagin has swelling on his brain, which caused pressure and led to his headaches. While Yagin remains hospitalized, as of this writing, doctors believe surgery won't be necessary.
The bout with Siver was originally scheduled for UFC 151 on Sept. 1, which was canceled. There's no word yet on who might replace Yagin on Dec. 8 in Seattle.