Mourinho insists De Gea does not have a price.
Asked to name a fee for the Spaniard, he said: “Zero. Because when a player is not in the market, his value is zero.
“Is David the best in the world? My goalkeeper is always the best goalkeeper in the world. I said that to my goalkeeper when I was coaching Uniao Leiria, a small club in Portugal.
“My goalkeeper is always the best in the world, but you understand what I mean. I don't want to be called disrespectful or arrogant and say that Courtois is not good or Manuel Neuer is not good. There are so many."
De Gea's mistake against Portugal in the World Cup when he allowed a Cristiano Ronaldo shot to evade his grasp led to criticism for the Spanish star.
But Mourinho defended him saying: “David played four matches in the World Cup and he made one mistake. With that mistake, his team didn't lose. He made that error against Portugal for one of Cristiano's goals.
“It was a technical mistake, but Spain equalised. They drew that game, won the next and drew the next and finished first in the group.
“Then they played Russia in the fourth game. They drew and lost on penalties. So for four matches and 390 minutes, David De Gea made one mistake and his team was not even punished by that mistake.
“But David knew perfectly he was not responsible for Spain being knocked out. He came back normal and in the States, he started working absolutely normal and here we have David."