Gray has been using the Aussie striker as a lone forward with Alan Smith back in midfield.
"Goalscorers do worry when the goals aren't coming, but I just said to him at half-time "keep going and the goals will come for you," Gray said.
"So I was delighted to see Mark Viduka score. Once he starts scoring goals to go with everything else he's got, we're dangerous.
"He's important to us in the system we are playing at the moment because he holds the ball up and he lets the two young wide boys support him and he plays in the boys in the middle of the park, plus he scored a great goal.
"His great strength is holding the ball up for others on the edge of the box. I have to say that since I have come back to the club, Mark has been terrific.
"Players need to use the ability they have and that comes from hard work and confidence.
"I am not looking beyond the next game, that's all that concerns me. When we played Chelsea last week I just said we'll look at the Fulham game and we'll do the same now with the Man City game. We'll take it each game at a time because they are all as important as each other."