Manchester City have plans to make a quality centre-half signing next month.
City boss Pep Guardiola reiterated his desire to land a centre half in January earlier this week and confirmed talks had taken place with sporting director Txiki Begiristain.
Martinez - capped four times for Spain - has spent his entire career at Sociedad and built a strong reputation for himself in La Liga.
The 26-year-old's release clause is £29 million, less than Jonny Evans would have cost had City managed to seal that deal in August.
Van Dijk, still a target for Liverpool after their failed summer attempt, would find it easier to slot in at the Etihad Stadium.