The 27-year-old marksman has fired four goals in 10 matches so far this season, while last term he struck 11 in 16 following his January arrival from Rangers.
"If we keep giving him the opportunity to score goals, he'll come back because he's a really good goalscorer," Moyes said.
"In the second half of last season he was the one getting one or two goals and getting us out of sight.
"At the moment we're missing his fire, but players go through different periods of form during the season.
"I've told him to keep going and that his goals will come.
"He's a really important player for us who made such a big difference last season.
"The team have improved so we're giving him chances to score. He will score, I have no doubt of that."