The Italy international added to a header from Sebastian Prodl in the 64th minute, with Watford holding on for the three points after Lukaku halved the deficit late on.
Deeney suggests it is a sign of big things to come from Okaka over the course of the season.
"The big man [Okaka] did fantastic today and that's his second start so he will build on it," the Watford captain told Sky Sports.
"We have a real powerhouse on our hands and he will cause problems."
Okaka admitted he has found a lack of playing time tough since his arrival from Anderlecht, but is confident he can continue to contribute positively.
Watford v Everton: Player Ratings
/en-gb/match/watford-vs-everton/2241908/ratings?ICID=MP_MS_6"It was a little bit difficult in the beginning because for a long time I didn't play, but it was a fantastic game and to win is so important," he said.
"I know my quality and my team-mates know my quality. In Italy [before the 2015-16 season] I was injured, but when I was in the picture everybody helped me."
On his skilful first goal, he added: "I have scored three or four times like that. It's amazing."