The European Commission president said he was not expecting the UK to "ask for extra time" but that he would not block a further extension if one was requested.
Mr Juncker admitted that he "cannot judge" how Mr Johnson will treat the crunch European Council summit later this week if no deal has been agreed between the EU and UK.
Under the terms of the so-called Benn Act passed by parliament last month, the prime minister must ask the EU for a further delay to Brexit if a deal has not been approved by 19 October.
However, Downing Street have said repeatedly that Mr Johnson will not request an extension "in any circumstances" and will instead fulfil his "do or die" pledge to deliver Brexit by 31 October.
Mr Juncker suggested that he was expecting Mr Johnson to stick to his pledge.
He told Austrian newspaper Kurier: "If the British ask for extra time, which they probably will not do, I would consider it unhistorical to refuse. But I'm sure I'm not going to kneel to ask for an extension. That's the British thing.
"From today's perspective, I cannot judge what Premier Johnson will do at the European Council this week. There is also a British law that forces Johnson to ask for extension. And what brings the British parliament to the final decision is not clear.
"I recently said that compared to the British parliament, an Egyptian Sphinx is an open book. So you have to wait, and because of England, drink tea."
Mr Juncker also said he regretted listening to David Cameron when the then prime minister insisted the EU should not intervene in the 2016 referendum campaign.
He said: "I do not attribute any causal guilt to the Commission and the EU to this referendum disaster, but I blame myself for listening Premier Cameron when he told me that the Commission should not interfere in the referendum campaign.
"So many lies have been served that no one in Britain has objected to. And the Commission could have proved with facts that much of what was said was wrong. That was a mistake."
Mr Johnson will update his Cabinet on Sunday afternoon on the status of negotiations with the EU. The two sides were locked in talks over the weekend after a breakthrough secured when the prime minister met his Irish counterpart, Leo Varadkar, last week.