Reilly told the weekly publication Wirtschaftwoche that restructuring charges were weighing heavily on the results.
"In total (for 2010) we are looking at a loss of about $2.0 billion or €1.4 billion," he said.
Opel has already reported a net loss of €1.2 billion in the first nine months of the year. It hopes to make a profit from 2012.
A few months ago, GM began a deep restructuring of the Opel brand involving a loss of some 8,000 jobs in Europe out of a workforce of 46,000, half of whom are based in Germany.
The measures will cost the company €1.0 billion this year and €500 million next year, Reilly said.
Opel, which operates under the Vauxhall brand in Britain, has been in difficulties for years and its market share has fallen.
This contrasts with the state of the parent company which returned to the stock market last week only 18 months after bankruptcy and expects to be profitable for the whole of this year, the first such annual result since 1994.