"Her hopes are to return to the Octagon as soon as possible but for now we ask for people to respect their privacy during this time," manager Brian Butler said in a statement Thursday.
Namajunas was scheduled to fight Jessica Andrade in the April 18 co-main event. UFC president Dana White said later Thursday that UFC 249 won't proceed on that date, after all.
ESPN has Namajunas ranked No. 5 pound-for-pound among women's MMA fighters, and Andrade is ranked No. 7. No replacement has been announced to fight Andrade.
Namajunas (8-4) has not fought since Andrade (20-7) knocked her out via slam to win the strawweight title at UFC 237 in May. The Wisconsin native had won three in a row before that, including two title-fight wins over Joanna Jedrzejczyk. Namajunas, 27, has lost only twice in six years.
Information from ESPN's Ariel Helwani and Marc Raimondi was used in this report.