The British prime minister suffered a serious blow to his credibility by losing his fight against Juncker, a veteran EU insider, and Cameron admitted it made the job of keeping Britain in the European Union harder.
Many in Britain -- where a long history of euroscepticism reached new heights with the victory of the anti-EU UK Independence Party in last month's European Parliament elections -- currently support his stance against Juncker, who is seen as too much of a federalist.
Some 43 percent of voters believe Cameron was right to try to block Juncker's appointment, against 13 percent who said he was wrong in a Financial Times/Populus poll released this week.
And Cameron could still achieve the major EU reforms he wants, including repatriation of some powers, depending on how he reacts now, according to analysts.
"The Juncker episode is clearly a substantial defeat for David Cameron, and without remedy, increases the risk of Brexit (a British exit from the EU)," said Mats Persson, director of the London-based think-tank Open Europe.
"However, it is far from the end of the story for sweeping European reform."
British newspapers were unanimous on Saturday that Cameron's defeat had increased the risk of the country leaving the 28-nation bloc, with The Times warning on its front page, "Britain nears EU exit".
But they were divided on whether Cameron or Brussels was to blame, with several saying he was right to stand in "splendid isolation" on a matter of principle.
Persson suggested that a perceived increase in the risk of Britain leaving the EU could prompt fellow leaders to swing behind Cameron, who he added must now work harder to outline his vision of reforms in Europe.
Key to Cameron's future situation will be who gets what as the rest of a large-scale shake-up of top EU jobs is negotiated at another summit on July 16.
Britain could be handed a key portfolio or one of its allies could take an important role such as the presidency of the European Council, the EU's political arm, currently filled by Herman Van Rompuy.
Cameron said Friday the key test for him of a candidate's suitability for a job was: "Do you get the need for reform and change in this organisation?"
Ironically, his ability to drive through change ahead of the 2017 referendum on Britain's EU membership will also hinge on his ability to build a relationship with Juncker, whom he has called "the career insider of Brussels".
"As president of the European Commission during and after the UK's 2015 general election, Juncker's stance on the UK's relationship with the EU, the prospects and modalities of re-negotiating the terms of membership -- and a possible referendum campaign -- will be significant," said Professor Richard Whitman, associate fellow, Europe, at the Chatham House think-tank in London.
Cameron insisted Friday he was fully committed to fighting for Britain to stay in a reformed EU, regardless of how hard that is.
Simon Hix of the London School of Economics said Cameron needed to deliver on two main points -- a reformed agenda for the single market and concessions on British opt-outs from certain parts of EU law.
"If he can deliver on those things, then eveyone will forget Juncker," he added.
But eurosceptics are not convinced, saying the argument over Juncker's nomination indicates a lengthy fight ahead for Cameron to secure reform.
"The battle over Mr Juncker was but the first skirmish in a long negotiation of a new relationship for the UK with the rest of the EU," veteran eurosceptic MP and former minister John Redwood wrote on his blog.
"If the rest of the EU continue to be so unsympathetic to UK requirements, more UK voters will draw their own conclusions about the desirability of our continued membership."