SoftBank Group, the biggest investor in WeWork owner We Co., is exploring ways to replace Adam Neumann as chief executive of the U.S. office-sharing startup, according to four sources.

The rare showdown between SoftBank and one of its biggest investments comes after We Co. postponed its initial public offering last week following pushback from prospective investors — not just over its widening losses, but also over Neumann's unusually firm grip on the company.

The move was a blow for SoftBank, which was hoping that We Co.'s IPO would bolster its profits as it seeks to woo investors for its second $108 billion (¥11.6 trillion) Vision Fund. It invested in We Co. at a $47 billion valuation in January, yet stock market investor skepticism led to the startup considering a potential valuation in the IPO earlier this month of as low as $10 billion.