Newcastle are likely to be after a central midfielder this summer.
The 22-year-old is closing in on a move to the Spanish side and now it looks to be all but completed, with Benitez telling the Chronicle that he is disappointed to let him go - but "it was something we couldn’t control because he decided to go."
That leaves a gap in the middle of the park, with only Jonjo Shelvey, Isaac Hayden and Mo Diame to choose from. Jack Colback and Henri Saivet are expected to leave the club either on loan or permanently this summer.
With Hayden struggling to show Premier League quality last term and Diame's long-term future at Newcastle a doubt with only 12 months remaining on the three-year deal he signed in 2016, a new partner for Shelvey must be considered.
England international Shelvey is renowned for his ball-playing attributes and ability to dictate the tempo of a match from deep, but also the ability to get forward and shoot from 25 yards out.
He needs a central midfielder that is willing to do the dirty work in order to give the ex-Liverpool and Swansea City man enough time with the ball. Benitez must sign a dogged, hard-working midfielder with lung-busting stamina.
That should convince the Spanish boss to look at West Ham United star Cheikhou Kouyate, after Sky Sports' live transfer blog (09/07; 17:39) claimed that the Senegal international was available for just £10million.
With Merino's departure likely to bring in a figure close to that mark, Newcastle could do a lot worse than bringing the former Anderlecht star to St. James' Park.
His aerial ability and tough-tackling style of play would see him win the ball back and feed Shelvey, who can then mastermind Newcastle's attacking play. He would be Shelvey's perfect midfield partner.