Ronaldo has scored eight goals in 11 league games this term, but has only found the net in four matches and has been criticised for his form as the Clasico clash with the European and Spanish champions approaches on November 21.
But Nacho has refused to criticise his Portuguese team-mate and expects an imminent return to form.
"We're talking about the leading goal scorer in the history of Madrid," Nacho said.
"I wouldn't just back him, I have complete faith in him. He'll score more goals for Madrid. When he doesn't score he isn't happy but his ambition is huge. He can't be happy if he hasn't scored."
Rafael Benitez has been criticised after some underwhelming performances, with his side losing 3-2 to Sevilla in their latest game.
Reports have emerged of problems and unrest in the dressing room, but Nacho maintains that the players support the coach.
"We've just lost a match and all the bad stuff is brought out," he said.
"We've lost one game and we're doing things well. The players are united with the coach. We'll go into the Clasico fine. Things get taken out of perspective by we're confident we'll have a good>