The Chelsea forward is the Premier League's leading scorer this season, but he has failed to find the back of the net since making his debut for Spain earlier this year.
Alcacer told Marca: "He will score soon, I have a very high opinion of him.
"These good and bad patches are quite normal and soon his first goal will come. [Casillas] is an example to follow. You can't judge him on a single game."
Spain were beaten 2-1 by Slovakia earlier this week.