However, Olli Rehn said that there were other ways to keep Greece’s aid programme going, such as extending the repayment schedule on existing loans.
His comments in a Finnish newspaper came after Germany’s finance minister said Greece would need another bailout.
Greece has received two bailouts totalling about 240bn euros (£205bn).
Greece’s troika of lenders – the European Commission, the European Central Bank and the International Monetary Fund – will review the aid programme in the autumn.
“Then we will also review Greece’s programme’s possible continuation and financing. The debt sustainability can be improved, for instance, by extending the loan periods,” the Helsingin Sanomat newspaper quoted Mr Rehn as saying on Wednesday.
However an ECB board member, Joerg Asmussen, was in Athens on Wednesday to discuss the country’s fiscal reforms with the Greek prime minister and finance minister.
A condition of the current bailout deal is that Greece makes cutbacks and restructures its economy.
The question of whether more loans are needed depends on the success and pace of this restructuring.
On Tuesday, Germany’s Finance Minister, Wolfgang Schaeuble, said: “There will have to be another programme in Greece.”
The comment came at a sensitive time, as Germany prepares for elections and amid concerns in the country about the size of its contribution to the bailout of EU countries.
On Wednesday, Sigmar Gabriel, head of Germany’s SPD party, called on Chancellor Angela Merkel to “come clean” on whether the country would have to contribute more funds to Greece.