Cristiano Ronaldo has scored just twice in 10 LaLiga matches this season as a stumbling start by Madrid has left them eight points adrift of leaders Barcelona.
However, Ronaldo leads the goalscoring charts in the Champions League with eight in five games and Zidane expects that form to soon return on league duty.
"He is so good that when things don't go as he is used to then people start to talk about him," Zidane said on Tuesday ahead of Real's Champions League dead rubber against Borussia Dortmund. "He will keep trying until the goals come. Not long ago, only last year, he had a phenomenal season."There is a long way to go, we are only nearly halfway though the season. Be careful with Cristiano."
With Karim Benzema also failing to find the net regularly, a lack of goals has blighted Real's season so far.
Until September, Zidane's men had scored in 73 straight games.
However, they have now drawn blanks in three of their last nine La Liga outings against Real Betis, Atletico Madrid and on Saturday in a 0-0 stalemate at Athletic Bilbao.
Zidane, though, rejected widespread suggestions, most recently from Valencia coach Marcelino Garcia Toral, that Madrid's squad is poorer this season for the loss of Alvaro Morata, James Rodriguez and Pepe in the close-season.
"It is true that we need to score more goals because, being eight points behind in La Liga, the situation isn't ideal," added Zidane. "But I am calm because I know that sooner or later that will change. No matter what people on the outside say, possibly they aren't seeing what I am. "We also can't forget what this team has achieved. I certainly haven't."
With Madrid already assured of second place in Group H behind Tottenham Hotspur, Zidane is expected to heavily rotate his side against Dortmund ahead of a busy end to the year for Real.
The European champions host Sevilla on Saturday before jetting off to Abu Dhabi for the Club World Cup and then face Barcelona at the Santiago Bernabeu on December 23.