The 27-year-old, who joined Boro from Real Madrid in 2014, will finish the season in the Spanish second division.
Middlesbrough goalkeeper Tomas Mejias joins Rayo Vallecano on loan
Middlesbrough announce that goalkeeper Tomas Mejias has left the club to join Rayo Vallecano on loan.
Derby announce signing of De Sart>
You May Like
Recomended from Sports Mole
Follow @SMMiddlesbrough to get every Middlesbrough headline from Sports Mole, and follow @SportsMole for the latest breaking news stories