Barca suffered a shock La Liga defeat at home to tiny Hercules on Saturday, but Guardiola blamed himself ahead of Tuesday's Champions League Group D opener against Greek champions Panathinaikos.
"It's impossible for me to doubt these players," the 39-year-old told a news conference.
"We could have our worst season ever and I still wouldn't doubt them. The other day (against Hercules) maybe I failed to help them do their jobs."
Guardiola's decision to rest World Cup winners Xavi, Sergio Busquets, Pedro and Carles Puyol against Hercules backfired badly.
Even with their Spain team mates David Villa and Andres Iniesta and Argentine World Player of the Year Lionel Messi on the pitch, the 2009 Champions League winners looked toothless.
Guardiola, who led Barca to an unprecedented six trophies in 2009, said he had no doubt the hunger for more titles was still strong.
"I know them and they are hurt that they lost," said the former Barca and Spain midfielder, now in his third season as coach of the Catalan club. "I know that they want to compete well and remain at the level they are capable of."
Iniesta acknowledged that the players needed to pull their socks up.
"The Champions League is a very demanding competition and you are only allowed a very few errors," he said.
"They (Panathinaikos) have players with a lot of experience who have played for good teams.
"It's the first match of the group phase and it's always important to win it. We are taking the match very seriously."
Danish side FC Copenhagen and Russia's Rubin Kazan complete Group D.
