When asked which of Neymar and Mbappe would be harder for PSG to keep, Herrera said: "I don't know, because I don't know what they earn or the income they generate for the club, which is still a company.
"But I do know that both are key in the project and I see them very happy and involved with PSG."
Herrera added that PSG are more accustomed to buying star players than selling them.
"You have to take into account a fundamental aspect, and I have discussed it with Kylian, that PSG is not a club that sells its stars, on the contrary, it buys them," he said.
"It has enough economic muscle to support both of them."