The tiny Alicante-based club, returning to the top flight after more than a decade, inflicted Barca's first home defeat in the league since May 2009, Paraguay striker Nelson Valdez scoring either side of halftime at a stunned Nou Camp stadium.
At a post-match news conference, Guardiola dismissed suggestions the state of the pitch or tiredness after international duty were to blame for the reverse, which spoiled the festival atmosphere on the National Day of Catalunya.
"It wasn't the pitch or the FIFA virus, our opponent was simply better," the 39-year-old said.
"We just need to take on board that things are not that easy and hopefully that will help us focus better," he added.
"Our players are of an exceptionally high level and I will not let them drop their performance for a moment."
Guardiola may be wishing he had deployed World Cup-winners including Xavi, Sergio Busquets and Pedro from the start at a sun-bathed Nou Camp.
But even after Xavi and Pedro were introduced at the break Barca struggled to break down a determined Hercules side and former Juventus striker David Trezeguet should have made it three for the visitors late on.
"This doesn't mean we have to worry," Guardiola said, adding that the team had to focus now on Tuesday's Champions League group match at home to Greek side Panathinaikos.
"We are facing a difficult year, a tough year, so we have to pick ourselves up quickly both mentally and physically.
"We were lacking in rhythm today. They closed us down very well and it's hard to attack teams that work together so effectively."
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