David de Gea is being singled out for criticism at international level because "somebody very powerful" - namely, Iker Casillas - wants to replace him as Spain's goalkeeper, according to Jose Mourinho.
De Gea's position in the Spanish national side has come under increased scrutiny this year following a difficult World Cup campaign.
The Manchester United goalkeeper was blamed for his national side's group stage draw with Portugal and the penalty shoot-out round of 16 defeat to hosts Russia.
Amid questions over De Gea's place, Casillas said last week he would "happily" play for the Spanish national team again.
The 37-year-old Porto goalkeeper, who captained Spain's 2010 World Cup winners, is yet to announce his international retirement despite not receiving a call-up since 2016.
Mourinho came to De Gea's defence on Friday, describing him as "the best goalkeeper in the world", and suggested that his player is only criticised in his home country because Casillas wishes to return to the national team.
"Why criticism in Spain? Do you know? I know," Mourinho said. "Because there is somebody very powerful that is thinking about jumping again but I think it’s very difficult to jump when you have the best goalkeeper in the world in front of you."
Mourinho did not reference Casillas by name, though all but confirmed his identity by answering a series of follow-up questions.
The United manager confirmed he was speaking about a player who is "in the media". Casillas' most recent remarks about returning for Spain were made while appearing on television, while Sara Carbonero, his wife, is a leading Spanish sports journalist.
Mourinho then said that the player in question had once played for Real Madrid and when asked if he "liked him", he replied: "Yeah, I like him. I was champion with him."
Casillas and Mourinho won La Liga together at Madrid in 2012, though a subsequent breakdown in their relationship contributed to Mourinho's departure from the Santiago Bernabeu a year later.
Last week, in the aforementioned television interview, Casillas expressed regret that he did not "take the bull by the horns" and confront Mourinho during their difficult period together at Madrid.
Mourinho responded on Friday, in quotes reported by the Portuguese newspaper Record. The United manager insisted Casillas did challenge him, yet in an indirect manner.
"It seems like an interview with someone who is at the end of his career," Mourinho reportedly said. "But when it comes to me when he says he has never confronted me, that is not the truth.
"He did it in a way that no one does better than him; secretly."
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