The 30-year-old central defender is into the final year of his contract with the Portuguese side, meaning that he is free to talk to foreign clubs over a bosman move next summer in January.
The defender fits the criteria that Koeman was looking for before last week's transfer deadline: an experienced left-footed central defender.
The former Spain U21 international can also fill in at left-back and even in defensive midfield, offering Koeman vital versatility.
English publication 'The Daily Mail' reports that Everton eqnuired after the Spaniard this summer but baulked at Porto's excessive demands.
Marcano made 46 appearances in all competitions for the Portuguese side last season and has captained that side in all four of their games so far this time out.