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Louis van Gaal has warned Gary Neville he faces a "very difficult" job reviving Valencia's fortunes.
The former Manchester United captain took his first job in management on Wednesday when he was appointed head coach of Valencia.
The Spanish side have won the Primera Division six times, but have fallen on hard times of late.
The club has been ravaged by political in-fighting and they sit ninth in the league - having finished fourth last season.
Van Gaal, who has managed Barcelona twice, believes former right-back Neville will find it hard going from TV pundit to manager of one of the most demanding clubs in Spain.
"Valencia is a very big club in Spain with a lot of history," the United manager said.
"They have won a lot of titles also, so it's a big step, a big challenge for him.
"I am very happy but also curious how he shall do that."
Van Gaal was not fluent in Spanish when he arrived at Barcelona in 1997 but won back-to-back league titles with the Catalan giants and the Copa del Rey before returning for a brief and less successful period in 2002.
Having brother Phil on his coaching staff will help Neville, but Van Gaal believes there are still plenty of obstacles for the 40-year-old to overcome.
"He doesn't speak the language so it's very difficult," Van Gaal added.
"He has his brother, he speaks fluent Spanish, but still it's a big challenge and I wish him a lot of luck because he needs luck."
When asked if the language barrier would prove to be Neville's biggest obstacle to success, Van Gaal said: "No, you have also culture dimensions, that is also difficult to understand.
"I have to adapt also as a manager to the culture of England and these dimensions are not always fitting in your way of thinking but you have to do it."
Neville's first match in charge will come next Wednesday when Valencia host Lyon in the Champions League.
Phil Neville and interim head coach Voro will take charge of Saturday's home game against league leaders Barcelona.