Fernando Morientes warned Real Madrid another club would snap up Monaco star Kylian Mbappe if they fail to do so.
Mbappe, 18, is widely expected to make a record-breaking move in the off-season, with Madrid linked to the forward.
Morientes, who won two LaLiga titles and three Champions League crowns with Madrid, said the teenager would end up elsewhere if his former club were unable to complete a deal.
"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 said that would 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."
