The Colombian international joined the Nerazzurri in January and he finally made his debut last weekend following his struggles with injury.
"The option to sign Guarin outright is set at 13.5m," Branca told O Jogo. "That is the fee agreed with Porto.
"We're very pleased with Guarin so far and he did well in his debut against Genoa on Sunday. However, there is some time to go before we decide whether or not to keep the player."