The Prime Minister is attempting to secure one of the Commission's key economic portfolios - such as trade, internal market or competition - for his nominee Lord Hill of Oaresford while meeting fellow EU leaders for dinner in Brussels.
New Cabinet minister Michael Fallon said it would "certainly help us" if the peer took one of these roles, and rejected claims Lord Hill lacked the profile needed to win respect in Europe, insisting he was "an effective political operator".
But German media reported the president of the European Parliament Martin Schulz had told German radio: "I can't imagine that Hill, with his radical anti-European views, which he is supposed to have, will get a majority in the European Parliament.
Mr Schulz said that a rejection of Lord Hill's nomination by MEPs, who vote on the new Commission headed by Jean Claude Juncker in September, could not be excluded.
"It remains to be seen whether Mr Hill is unprejudiced towards us, and on that will depend whether he gets a majority," he said.
The Prime Minister's official spokesman pointed out the European Parliament has the opportunity to approve or reject a new Commission "as a whole, as a slate, not as individuals".
The spokesman said: "The Prime Minister is very confident he will command wide respect in Brussels."
Asked whether Lord Hill had "radical anti-European views", the spokesman added: "It is no secret Lord Hill shares the Prime Minister's views in terms of the need for Europe to reform and changing Britain's relationship with the EU as part of that."
The spokesman said Mr Cameron "thinks the appointment of Lord Hill is the right one and he has the expertise that will make him an excellent UK Commissioner".