The French striker has scored just two goals in his last 18 games for the Spanish side and was substituted just past the hour mark in Saturday's 2-1 home defeat to Atletico Madrid.
However, Zidane has backed his countryman to overcome a disappointing run of form.
"He is the main striker at Real Madrid even though he hasn't scored much lately, but his critics should always get behind him," he told RMC Sport.
"Karim should score for both Madrid and France and he is conscious of that. He knows what kind of player he is and how much is expected of him.
"Last year I was a lot closer to him, but not so much this year because I have been a lot busier. In the next match he will score and everybody will then move onto something else."
Zidane acted as assistant to Madrid boss Carlo Ancelotti last season, but took charge of the Castilla side at the beginning of this season.