(CNN) Champions League holder Liverpool was brought crashing back down to earth, as it started the defense of its title with a 2-0 defeat away to Napoli.

Dries Mertens' penalty and Fernando Llorente's late goal following an uncharacteristic Virgil van Dijk error consigned the Reds to a fourth successive defeat on the road in the Champions League group stages.

Liverpool came into the match in fine form and with an air of invincibility, winning all five of its Premier League games so far this season and showing no signs of a Champions League hangover.

But defending the European crown is not easy; only one club in the competition's history has successfully retained its Champions League title, Real Madrid on two occasions.

However, Liverpool will certainly not be panicking -- perhaps some will even see this as a good omen. On its way to lifting the title last season, Jurgen Klopp's side narrowly escaped the group stages despite losing all three of its matches away from home.

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