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.
"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."
"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.
