The 16-year-old came off the bench to score deep into stoppage time in Monday's Premier League encounter at St. James' Park as the champions snatched a 3-2 victory after 10-man Newcastle had battled back from 2-0 down.
Van Dijk told Sky Sports: "It is a dream debut for him. It was a perfect technique. We stayed calm towards the end as well, tried to find the right solution in order to score a goal, and we did it. I'm very pleased for Rio."
With his late strike, Ngumoha, who turns 17 on Friday, became the fourth-youngest goalscorer in the Premier League, behind James Vaughan, James Milner and Wayne Rooney.
Information from PA and ESPN's Global Sports Research contributed to this report
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