The shares finished down 12% yesterday, their lowest since March 2009.
The Japanese company has lost about $2bn (€1.83bn) of market value over the past two days.
Toshiba has been dogged by an accounting scandal, prompting the company to consider third-party alliances for some units and a restructure of the business that makes TVs and PCs.
It is considering options such as listing the memory chip division or selling a majority stake in a medical equipment unit after restating earnings across seven years.
The projected net loss for this fiscal year includes 260bn yen in taxes because of a reversal of deferred income tax assets.
The forecast doesn’t include possible impairment of goodwill and fixed assets at the company’s nuclear power systems.
Toshiba was cut to Ba2, two levels below investment grade, from Baa3, Moody’s said.
Moody’s questioned Toshiba’s ability to fund an extensive overhaul and cited uncertainty around its ability to improve its operating performance.