The 27-year-old has already scored 44 goals for Porto since joining them Mexican side Chiapas in 2012, but has a £35m release clause in his contract and is also cup-tied in the Champions League.
Tottenham also took a firm interest in buying the forward last summer before buying Roberto Soldado from Valencia instead and are still monitoring the situation, while he has also been linked with Arsenal, Liverpool and Manchester United, says the London Evening Standard.
Martinez said: "The years are passing and one day I hope to make a good transfer. 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
"Napoli were one of the clubs who tried to negotiate (last summer), but Porto didn't want to hear anything about it because they wanted me to stay. It is true that two of the clubs interested in me were British, but neither passed the first step of negotiations."
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