The Nigeria international has struggled so far this season, managing just one goal for Mazzarri's side after starring last term.
His 18 strikes in all competitions was the highest total of any Hornets player last season and Mazzarri is sure the forward will start finding the back of the net again regularly soon.
"He still has confidence because we as a team have confidence in him," Mazzarri told Daily Star.
"They are periods where players are more in form than others. But we believe in him and it is just a matter of him scoring."
Ighalo was only named on the bench last weekend for the 1-0 defeat to Middlesbrough as Mazzarri opted for his international colleague Isaac Success up front instead.
However Ighalo had been back in Nigeria on compassionate leave in the week which may go some way to explain the decision, and he will hope to be back in the starting line-up against Swansea on Saturday.