Mbappe, 18, is widely expected to make a record-breaking move during the summer transfer window, with Madrid among those being linked with the forward.
"When you talk about these types of figures, it's usually because the player has generated a certain level of expectation. If you don't buy, another will," he told Efe.
"It's difficult to digest, but when a player is tempted by so many clubs, it's because he has a lot of talent."
Mbappe scored 26 goals in all competitions in 2016-17, with Paris Saint-Germain and Arsenal among numerous clubs believed to be interested in his services.
It may take a world-record fee to sign the teenager and Morientes says that will put pressure on the France international.
"He is just a kid, still in the process of maturing," he said.
"The weight of responsibility will be on his shoulders."