FRANKFURT — Maybe this is why central bankers typically don’t like to say much.

In what seemed a rare communications blunder by one of Europe’s most politically polished technocrats, the president of the European Central Bank, Mario Draghi, set off a market rebellion Thursday as investors who were apparently hoping for a bold euro rescue plan instead heard a general statement of good intentions.

Mr. Draghi signaled that the bank, in what would actually be a marked departure from current policy, was willing to start buying government bonds to hold down the borrowing costs of troubled euro zone countries like Spain. But the bank put the onus on political leaders to move first, and left open some crucial questions about how quickly and forcefully it would seek to tame unruly financial markets.

Coming almost exactly a week after Mr. Draghi incited a market rally by pledging to “do whatever it takes to preserve the euro zone,” his admission Thursday that whatever it takes might actually take weeks or months resulted in a Draghi rout.

Stocks indexes started falling quickly and deeply in Europe almost as soon as Mr. Draghi began speaking at a news conference. And the downdraft carried over to Wall Street after the American markets opened. Borrowing costs for Spain and Italy, as measured by the yields on their bonds, spiked back up almost to the levels they were at before the Draghi rally started a week earlier.