Bankers were generally taken aback by what was termed the "unilateral announcement" this week by Mexican Finance Minister Jesus Silva Herzog that Mexico will not repay any of the principal that comes due on its debts for the rest of this year and through 1983, sources said yesterday.

Mexico, which owes about $80 billion to foreigners, already has put off principal payments on all the public debt that comes due in the 90 days to the end of November, and had announced that it would like to stretch out later repayments.

Silva Herzog today denied a report that an agreement had been reached with private bankers to delay repayment of part of Mexico's debt another 15 months, United Press International said.

"I don't think any commercial banker really expected that he was going to start getting principal repayments again at the end of 90 days," one U.S. banker commented yesterday. Nevertheless, banks hope for a quid pro quo from the Mexicans for the bankers' agreement to reschedule debt, and were surprised by Silva Herzog's announcement, he said.

Protracted negotiations are expected before the banks formally approve any long-term postponement of principal payments, one banker commented. But he said that, if the Mexicans refuse to keep up principal payments next year, the banks will have little option but to accept this.

Recent swift changes in Mexico -- including nationalization of the banks, imposition of strict exchange controls and a new central bank governor -- have increased bankers' fears that it will take some time to deal with Mexico's financial crisis. International officials and commercial bankers are anxious that Mexico reach agreement with the International Monetary Fund on an economic program and three-year loan.

However, it seems increasingly likely that such agreement will come only after the change of government in Mexico which is due Dec. 1, sources said. Silva Herzog, now in Toronto for the annual meetings of the IMF and World Bank, repeated this week that Mexico hopes to have a final deal with the IMF by October or early November. However, one monetary official commented that any lasting agreement must be made with the new president, Miguel de la Madrid Hurtado, rather than with the present administration.