|Could he merrily wave the EU goodbye?|
There are good reasons to doubt the specifics of the report, because the source probably had ulterior motives: to damage David Cameron's campaign to dislodge Juncker. The theory being, the prospect of a Prime Minister "making" a threat would so upset EU sensibilities nobody would dare stand with Cameron's campaign.
Having said that the story does have two elements of truth to it.
Firstly, all polling shows that EU reform is the decisive factor in determining whether Britons would vote to stay in the EU. David Cameron could easily have pointed out that Juncker's appointment - an obvious example of non-reform - would damage the chances of an 'In' vote. But is that a threat - or a fact? Possibly, if David Cameron was the only person planning to vote in his planned referendum, but he is not. So, if it is a threat, it is a collective one from the British people.
Secondly, David Cameron has studiously avoided saying he could ever campaign for an 'out' vote - and been criticised for it in the UK. However, with Juncker's appointment, this is becoming a much more difficult position to sustain. If this continues, i.e. if the EU actively goes the wrong way, there would be a lot people in the UK saying it's not credible for him to say he would still recommend 'In'. (Open Europe's Christopher Howarth set out some reason's earlier here.)
The outcome of the Juncker saga may be that Cameron has no choice but to say he's willing to recommend "Out". But hey, the nature of the referendum is that it won't be his choice anyway.