The automaker, which emerged from bankruptcy in July, muddied the waters in the debate over whether it should get state aid when its third-quarter results revealed it had nearly $43 billion (30 billion pounds) in cash at the end of September.
Germany -- home to over half of Opel's 50,000 staff -- has given mixed signals on aid since GM's U-turn on the Magna deal.
EU Industry Commissioner Guenter Verheugen said on Monday that without state aid the revamp could not work.
GM's Reilly had travelled to Brussels to meet officials including Verheugen, Kris Peeters, the premier of Flanders, where Opel also has a plant, and EU Competition Commissioner Neelie Kroes.
"General Motors made one point very clear, 100 percent clear, the restructuring plan could only be achieved when European member states with Opel plants give some financial help," said Verheugen.
(Reporting by Tom Kaeckenhoff and Erik Kirschbaum, writing by Michael Shields and Helen Massy-Beresford; Editing by Jason Neely)