The 21-year-old - a summer arrival from Borussia Dortmund - hit the only goal of the game, and his first for the Magpies, in the 86th minute of the encounter at St James's Park.
The result got the side back to winning ways after a run of two draws and a defeat and, rather than accepting personal plaudits after the game, Merino insisted that the points are all that matters.
"It's great, the winning feeling, so I'm really happy for the result and my first goal. The most important thing is the three points," he told reporters.
"This is a really hard team to play. They are really compact and strong. We did a great effort physically, but in the second half we had more of the ball. At the end, from a set piece, we have our three points."
Next up for Rafael Benitez's side is a trip to Burnley in nine days' time.