Duke's 75th minute winner came when the Mariners were on the back foot after Sydney's Richard Garcia had cancelled out Kim Seung-Yong's superb opener.
Sydney have now lost three times to the Mariners this season and the result moves Central Coast to fifth spot on the ladder, jumping the Sky Blues.
Facing pressure with his side having lost five straight in all competitions, Moss had targeted 23-year-old Duke as one player who needed to improve.
"Fantastic," Moss said of the winning strike.
"I just said to him in the dressing room that's more like the Mitchell Duke that we all know.
"My constructive criticism of Dukey (was he) is not protecting the ball enough in general play and trying flicks around the corner when his back is to goal.
"Tonight I though he stood tall against two very good centre-halfs, two very big centre-halfs and two very experience centre-halfs.
"I thought he played really smart football, bought the midfield into the game and was always dangerous on the counter and took his chance very well to win the game."
After squandering a golden opportunity seven minutes before the winning strike, Duke showed composure beyond his years to deliver the crucial strike.
"Dukey obviously missed that one-one-one at one–all and he came to the sidelined and I said 'forget that, that's done but you'll get our next chance' and he took it so well," said Moss.
Injuries are now a becoming a concern for the Mainers but Moss isn't concerned over a nasty head clash Josh Rose suffered with Garcia in the second half.
"Well Rosey lost half his brain that came out in the gash in his head," Moss joked.
"He has got a few stitches in his head but we don't think there is any confusion there so that's a good sign.
"With Eddy Bosnar we will have to assess him tomorrow but he felt a bit of a strain (in his groin) when he lunged for the ball so that's something we will have to treat and look at the long-term… we won't take any risks with him."
Midfielder Nick Montgomery and striker Matt Simon were both missing on Saturday night after featuring on Tuesday in the ACL, however Moss expects them to play next weekend with their absence more to do with managing nagging injuries..
"Monty has had a bit of a calf issue for the last couple of weeks and played through it, I thought this was a game we could afford to bring young Caceres in who as fresh – only played 10 minutes over in Korea so that decision was mine to make," he said.
"Simo could have played at a pinch if it was a grand final but we have got the big picture in mind and we want to keep all our players on deck."
