Atletico Madrid's signing of Angel Correa could be in doubt after the Argentine's medical detected a heart problem that will require an operation.
Correa, 19, is due to join Atletico from San Lorenzo for an initial fee of around six million euros, with the forthcoming deal announced by the Spanish club late last month.
However, after his medical identified the issue he will now undergo an operation in Miami on Wednesday.
An Atletico club statement said both clubs were co-operating, and that no further complications or problems for Correa were expected once the operation had been carried out successfully.
However, it suggested that the transfer had yet to officially go through, and that another medical would be required before Correa could complete his switch.
He was to have been allowed to remain at San Lorenzo and play the rest of the Argentine club's Copa Libertadores campaign after the World Cup, but that plan has now been shelved.
Atletico president Enrique Cerezo told AS: "It would be an error to be worried about whether or not Correa can play in the Libertadores.
"The most important thing is the player's health. We will not know how long he is out for until after the operation -- it is premature to talk about it.
"We should all be thankful about the transfer, because due to the medical the issue was discovered. It is very common in many players."