"For a player like Mbappe, when you speak about the future, you don't need to say what he's going to be like in five years' time, in 10 years' time," Mourinho told beIN Sports.
"Just focus on now. He's absolutely incredible.
"I think going by his age and [and considering] the age of Cristiano, of Messi, [both] over 30, Neymar is 27... when you go to the market level and age is also a factor, I think he's the most valuable player in the world.
"In a hypothetical transfer, he's the most expensive player in football now.
"In terms of his qualities, [he is] unbelievable. One word is enough. It means everything."