The 26-year-old Colombian is reportedly the latest target for Napoli coach Rafael Benitez, who has reinvested part of the 64million euro fee paid by Paris Saint-Germain for striker Edinson Cavani by signing Gonzalo Higuain and Raul Albiol from Real Madrid, and Dries Mertens from PSV Eindhoven.
While Martinez's agent Manuel Manzo insists the player is eager to continue his career in Serie A, he stresses Porto are unwilling to part with a man who scored 26 league goals in his first season in Portugal.
He told Sky Sport Italia: "There's an agreement with the player, but we're missing one with Porto and the club don't want to sell him.
"The only way it could happen in my opinion is that they pay the 40million euro release clause, otherwise he won't leave.''
Primeira Liga champions Porto are increasingly keen to hold onto their key players after watching midfielders James Rodriguez and Joao Moutinho join big-spending Monaco in May.