Real, fifth in the league and trailing leaders Barcelona by nine points, have a lengthy list of injuries and suspensions but have been lifted by Tuesday's sacking of Bernd Schuster and the appointment of the former Sevilla and Tottenham Hotspur boss.
"I don't deny there is a feeling around here that we can score lots of goals against Real Madrid but we must not forget that it is the league champions visiting," Guardiola said at a news conference.
"They are coming to defend their title and you do not beat the champions 5-0 or 6-0. We start at 0-0 on Saturday and after we will see what happens."
Guardiola played for the Catalans in 10 "Clasicos" against their arch rivals from the Spanish capital and this will be his first as a coach.
He said he hoped there was no desire among his players to avenge Real's 4-1 win last year at the Bernabeu when they had already clinched the title. "That is in the past and is of no help now," he said.
Speaking at a separate news conference, Ramos said the players were extremely upbeat about taking on Barca and it had not been difficult to motivate them.
"We need to climb back up the league and we are totally convinced of our chances of winning," he said.
"The beginning of the match will be very important. If we concede early it could sap our confidence but even if that happens we still have a chance."
Ramos insisted the team would be going all out for victory, although he said he might play five in defence to try to counter the threat of Barca's dangerous forwards including Lionel Messi and Samuel Eto'o.
"I am only going for the win," he said. "The word draw is not in my vocabulary."
Real have been boosted by the return of Dutch international midfielder Wesley Sneijder to the squad, although Ramos said it was not yet clear whether he will play and medical staff were still assessing his fitness.
