While the Standard & Poor's 500 Index is recording its best monthly rally in 17 years, Roubini predicted it will not be sustained as the U.S. economy will continue to contract through this year and investors will start "discriminating" between solvent and insolvent financial companies.

"People are going to be surprised to the downside," Roubini said.

The government is conducting stress tests of banks to determine how much more capital each will need. Roubini said once those were completed it will be evident that some banks will need to be taken over and have their good and bad assets separated before being returned to the private sector.

Geithner's Plan

Critics of Geithner's plan including Krugman, a professor at Princeton University, say the government should take over banks loaded with devalued assets, remove their top management, and dispose of the toxic securities. Sweden adopted the temporary nationalization approach in the 1990s.

Roubini, who also runs his own economics consultancy, estimates a total of $3.6 trillion of loan and securities losses in the U.S., including writedowns on $10.84 trillion of securities and losses on a total of $12.37 trillion of unsecuritized loans.

With "deflationary forces" lingering for as long as three years, Roubini said U.S. government bond yields were going to remain relatively low and that American house prices would fall as much as 20 percent more in the next 18 months. While the dollar will benefit as investors seek safe havens, it will ultimately decline as the U.S. trade deficit has to shrink, he said.