It means the 24-year-old is likely to complete his move from Villarreal to The Emirates today in a deal worth around £13.5m.
Gunners ace Joel Campbell will go on the other way on loan for the rest of the season.
He looks set to take the No.5 shirt at Arsenal after Thomas Vermaelen vacated it over the summer.
The Brazilian has already passed a medical at The Emirates and Arsenal hero Charlie Nicholas thinks he'll be a big hit at the club.
Gunners great Thierry Henry has also given his backing to the deal.
Henry said: "Arsene knows what he needs.
"He obviously thinks the team is lacking a centre-back.
"Arsenal are going to get a young but polished player. I've heard he is a special talent."
