The 27-year old joined the Portuguese champions from Mexican side Chiapas in summer 2012, finding the net 43 times in 53 appearances for Paulo Fonseca's side following his move.
Jose Mourinho added to his striking options over the summer with the captures of Andre Schurrle and Samuel Eto'o, but the pair have only managed three Premier League goals between them so far this season.
Liverpool are also said to be amongst the clubs keeping tabs on the Colombia international, and Martinez has fuelled speculation by saying that he aims for a switch in the future.
"The years are passing and one day I hope to make a good transfer," he told Portuguese newspaper, Record.
"It clearly depends on Porto, a club that sells its players so well. Leaving or staying depends on Porto but I will only go if there's something concrete."