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But Herrera said both forwards looked happy at PSG, who claimed a third straight Ligue 1 title when this season was cancelled due to the coronavirus pandemic.
Asked who would be harder to retain if PSG had to sell one of the stars, the midfielder told AS: "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."
PSG landed Neymar for a world-record EUR222 million from Barca in 2017, while Mbappe joined permanently from Monaco the following year for a reported EUR135m.
Herrera said the French giants were more accustomed to buying stars 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."
Despite being close to a move back to Barca in the previous summer, Neymar is expected to stay at the Parc des Princes due to the financial climate in the world currently.
His former agent Wagner Ribeiro believes even the biggest clubs on the planet, who boast the deepest pockets, are going to find it difficult to finance big-money deals because of the conditions following the coronavirus pandemic.
With that in mind, Ribeiro doubts that an agreement can be reached that takes Neymar away from his contract at PSG and back to Catalunya.
He told Fox Sports: "I think Neymar will stay at PSG because the market is different.
"The economic world of football will change."
The Ligue 1 season has been cancelled - with PSG awarded the championship - and the new campaign will begin in mid-August.