The Spanish central midfielder, now playing for LaLiga side Real Sociedad, had his moments on Tyneside but lasted just one season with the Magpies.
Signed from Borussia Dortmund on loan in 2017, Merino's move was made permanent a few months later, but he found it tough to stay the distance.
Whilst he made 25 Premier League appearances in his brief spell at the club, on only eight occasions did he complete the full 90 minutes.
And because of that, he departed the following season to Real Sociedad where he is now one of the club's most important players.
He's led them to back-to-back seasons in the Europa League, as well as helping them win the Copa del Rey in 2020.
At the time, Merino said he made the right decision leaving Newcastle, but admitted that it was a difficult call to make.
"I felt at home there and I'm not lying when I say that," he said to ChronicleLive.
"I felt so welcome by the fans, by my team-mates, by the atmosphere. The city was amazing.
"I was enjoying just going to the city centre and having a pint or going for a walk and enjoying the people speaking nicely to you. I was feeling great.
"I look back on those times and it was a shame I could not play more games and enjoy the fans and the league a little bit more, but my mindset right now is it was a huge year for me. I learned a lot.
"I'm really sure I'm the player that I am now because of Newcastle and all the experiences that I lived there. I'm really happy that I lived that and I wish all the best for every Newcastle fan.
"The team right now are killing it so I'm really happy for them.
"Even now, when I watch the games and you see over 3,000 people travelling for an away game - it's crazy! You start to remember those times when you were playing away and you felt like you were playing at St James' Park.
"I still get goosebumps. It was a crazy year for me. I miss those days a little bit, but I'm really happy where I am right now."