Tottenham Hotspur manager, Mauricio Pochettino, has admitted that he might be powerless from stopping Harry Kane's move to Real Madrid, if he decides to leave.
Pochettino pointed to Philippe Coutinho's move to Barcelona, as an example of the lure of Spain's top clubs.
Barca and Madrid have a history of forcing even the biggest European clubs to sell their best players and Cristiano Ronaldo, Luis Suarez and Spurs pair Luka Modric and Gareth Bale have all moved from England to Spain in the past decade.
Kane has been heavily linked with a move to Madrid this season.
"I think it is a massive example or massive point to show how this business is, how difficult it is for the clubs to keep their best players," Pochettino said of the Coutinho transfer.
"Liverpool is one of the best clubs in the world and look when Coutinho wants to leave how difficult it is to keep your best player.
Liverpool wanted to keep [him] but it was difficult to keep him - and look what happened with Cristiano Ronaldo and Man United and [Zinedine] Zidane at Juventus, [Luis] Figo in Barcelona.
"It is a lot of examples that it is so, so tough to keep your best player if the player says he wants to leave.
"That is why it is so important how you care for your players - it is why you need to anticipate things. It is so important to work with the human side and then to provide the player the club and the team an exciting moment and try and build a project that they feel comfortable with and they are happy to be with you.
"Look, that is a good point to assess and talk.
Today when a player decides to leave, look what happens. If another club pays the type of money they pay - like Barcelona with Liverpool - how do you stop it? I hear what clubs say. It is so difficult, our job."