A 'no' vote in Sunday's referendum would dramatically weaken Greece's position in debt negotiations, European Commission President Jean-Claude Juncker said on Friday, declining to say whether creditors would re-open negotiations in that case. "If the Greeks will vote 'no', the Greek position is dramatically weakened," Juncker told a news conference launching Luxembourg's six-month presidency of the European Union. "Even in the case of a 'yes' vote, we'll have to face difficult negotiations," Juncker said.