The agent of Colombian international Guarin, 25, has admitted that he has spoken to the Serie A side, but also said that no formal contact has yet been made.
"With the Bianconeri there are the beginnings of negotiations, but nothing official," Marcelo Ferreyra told calciomercato.com.
"We are still at the stage of information exchange.
"At the moment their officials have only talked with us and not with Porto."
Juventus are currently second in Serie A, level on points with league leaders AC Milan.