Former Porto star Jackson Martinez can see Luis Diaz leaving in January.
Martinez believes the Colombia attacker's €80m buyout clause is within reach of several European clubs.
Chelsea, Everton and Newcastle United are interested in Diaz.
And Martinez told O Jogo: "If he continues like this, it could be difficult (to keep him).
"This clause will be easily paid.
"He must be followed by the whole world. Besides, he is sensitive as a person and I want him to continue in the best way."
He added: "Díaz is going to be the future James [Rodriguez] or the future Cuadrado, and I hope he stays that way."
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