Second placed Dortmund are currently on a five-match winning streak in the league, while the Karnevalsverein find themselves in 14th spot after winning their last three fixtures.
"We now have five important matches in two weeks," said Kovac to the BVB website.
"Mainz is the most important game for me because a win will help us to prepare for the upcoming games."
Dortmund will also be facing Atalanta in the Champions League two-legged playoff as well as fixtures against Bundesliga sides Bayern Munich and RB Leipzig in February.
"I am convinced that we have enough quality to win the game," added the Croatian.
"Mainz are fighting for survival. They put in good performances in Munich and Leipzig. This is not a walkover; this is a very well-prepared team."
The coach also confirmed that Karim Adeyemi and Marcel Sabitzer are expected to back in the squad after recovering from injuries while Yan Couto is doubtful. Emre Can and Filippo Mane remain unavailable.
Kovac has faced Mainz on two previous occasions as Dortmund coach, winning the first encounter at home 3-1 in March 2025 and the second clash and 2-0 away in September.
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