By William Joyce
The 24-year-old, who began his career as a forward but has since been converted to right-back, has quit his contract with the Spanish side after a disappointing 17th-place finish in the second tier last season.
Boro are in desperate need of a right-back and have failed to land top target Juanfran, of Real Betis, who boss Aitor Karanka worked with at Real Madrid.
Now they look to have won the chase to land Nsue after the player travelled to England for talks. The former Spain youth international, who now represents Equatorial Guinea at senior level, was linked with a move to Arsenal in 2010.
Karanka is aware of Nsue from his time as a coach within the Spanish youth ranks, where he also worked with new signings Kike and Tomas Meijas.