Juncker was chosen despite fierce objections by Britain, which regarded him as too federalist.
In other comments, Juncker urged the 27 EU countries — the entire bloc minus Britain, which wants to leave — to face its challenges with strength and unity, but admitted to “serious doubts” that its members shared the same goals.
“Has the time come for when the EU of the 27 must show unity, cohesion and coherence?” he asked.
“Yes, I say yes, when it comes to Brexit and (US President Donald) Trump… but I have some justified doubts that it will really happen.”
He added: “Do the Hungarians and the Poles want exactly the same thing as the Germans and the French? I have serious doubts.”