The Spaniard moved to St James' Park on a permanent deal in October after an initial loan move from Borussia Dortmund, as he impressed fans and Rafa Benitez alike with his technical ability and eye for a pass.
In the first half of the season he was a regular in games, but as the season progressed and Benitez looked to increase his side's defensive solidity, Merino became less and less involved in Premier League games.
Despite having signed a 5-year contract with the 'Magpies', Merino decided to make the move back to his native Spain because of his lack of opportunities on the pitch, with Real Sociedad believed to have paid a fee in the region of £10 million.
His compatriot Ayoze Perez formed a close bond with the player while they were at the club together, and took to social media to express his disappointment at Merino's departure.
July 12, 2018
While the club and Perez were disappointed to see Merino go, both will be ecstatic at the return of Brazilian winger Kenedy from Chelsea on a season-long loan deal.
Kenedy was instrumental, along with Perez, in the team finishing in 10th place in the Premier League last season.