Merino struck against his old club before Gabriel Magalhaes headed home a 96th minute winner.
Merino said afterwards: "I just said a long time ago but still big memories. Every time I come back here it's a great day for me. Also, all friends here, very happy to come and experience St. James' Park again now as an away footballer. A day full of emotions for me and to come back and score the equaliser is magical.
"Really happy (with his goal), that's something I put a lot of work in, headers to try and train every single day in the game, if you have a chance to have a header to have it as well as you do in training.
"Every single Premier League game's is an opportunity for us, not only to win points, not only to make a statement, but also to show everybody what this club is about, to show the character and the mentality we have. There's no better place than here, no better way then the way we have done it coming back from a really difficult situation.
"I'm really proud of everyone in there, not only the players, the staff and the fans who came here to support us. Just buzzing and ready to keep going."
On Gabriel's late winner, Merino added: "That shows a lot and not only set-pieces strategy but also mentality. Other teams if they get six/seven corners and they don't score, they start to lose belief in the qualities but for us it's a matter of keep doing what we do, doing it at the highest level and in the end the last one, last second, we scored and I'm really happy."