Ronaldo has scored eight of Madrid's 14 goals in the Champions League so far, to help the current holders reach the last 16 with a second placed finish in Group H, behind Tottenham.
No player has scored more goals in the Champions League than the Portuguese star, and he is also the top scorer in the competition this year, but he has just two La Liga goals to his name this season.
Ahead of Madrid's home clash with Dortmund on Wednesday, Zidane said: "Write Cristiano off at your peril.
Even though the side from the Spanish capital have already secured their place in the knockout stages of the Champions League, all the pre-match talk has been about Ronaldo's disappointing goalscoring form domestically, as Los Blancos trail Barcelona by eight points in La Liga.
"We have to give Cristiano a bit more support and show more respect," added the Real manager.
"He's so good that when things aren't going quite so well for him, people start to talk.
"The people who love him and Real Madrid know what he's doing. He's working hard until the goals start flowing again.
"Last season he did a phenomenal job. It's a long season and we're only halfway there."
Zidane also confirmed that the Dortmund game has come too soon for Bale, who suffered another injury during his comeback in the Copa del Rey game against Fuenlabrada last week.
"He still hasn't trained with us. He's out on the pitch doing bits, but not alongside the team," said the French coach.