Manuel Preciado, who was due to be unveiled as the new coach of Villarreal on Friday, has died of a heart attack.
Villarreal announced on Wednesday they had reached an agreement with Preciado to take charge of the club next season, with the 54-year-old set to be presented to the media on Friday.
However, Villarreal said in a statement on Thursday morning "The coach Manuel Preciado died overnight in Valencia as a result of a heart attack.
"The Cantabrian, who had been chosen by Villarreal to lead next season's project, was 54 years old.
"Villarreal want to express their deepest sorrow at the sad loss and want to offer their condolences to his family and all his relatives."
Spain coach Vicente del Bosque, preparing his team for the European Championship in Poland, said: ''We are all shocked,''
''We were having breakfast and we were all taken back by the news,'' he told Spanish radio Marca. ''He was a caring person and a great sportsman. He was a man of football that was an honor to coaching.''
Primera Division clubs and players paid tribute to Preciado. Champions Real Madrid wrote on their website: "Real Madrid regret the death of Manolo Preciado and wish to send their condolences to family and friends."
Barcelona also offered their sympathies, saying: "FC Barcelona want to pass on their condolences to the family and friends of Manuel Preciado. May he rest in peace."
Barca midfielder Cesc Fabregas tweeted: "I can't believe the bad news i just woke up with. Hugs for the family of Manolo Preciado. RIP my friend."
Sergio Ramos, who is with Fabregas in the Spain squad preparing for Euro 2012, also tweeted: "Football is mourning. I got up and the first thing I heard is the news about Manolo Preciado. Life is unfair and a great person has gone."
Gerard Pique, another member of Spain's European Championship squad, said on his Twitter account: "A big hug to the family of Manolo Preciado. One of the greats of this sport has gone. Rest in peace."
Preciado parted company with former club Sporting Gijon in January following six years in charge of the Asturian outfit.
Preciado took over Sporting in the summer of 2006 and in his second season led them into the Primera Division for the first time in 10 years.
His record of 232 official matches in charge at El Molinon is second only to Jose Manuel Diaz Novoa, who sat in the Sporting hotseat for 282 games.
A statement on the Sporting website read: "With deepest regret, Sporting Gijon wish to express their sorrow at this sad loss, for someone who formed an integral part of this club's history, and send their deepest condolences to his family in this time of grief."
Prior to his time at Sporting, Preciado was coach of the likes of Racing Santander, Levante and Murcia, while as a player he represented Racing, Real Mallorca and Alaves among others.
Sporting were relegated from the Primera Division in the recently-completed season, as were Villarreal.
One of Preciado's high points in charge of Sporting came against Real Madrid when, in April 2011, his side won 1-0 at the Bernabeu to inflict a first home league defeat on Jose Mourinho in more than nine years during spells in charge of Porto, Chelsea, Inter Milan and Madrid.
Earlier that season Preciado had a well-publicised war of words with Mourinho after the Portuguese had accused Sporting of gifting Barcelona three points after fielding a weakened team in a league match.
Tensions between the two eased though, and earlier this year Preciado said during a visit to Madrid's Valdebebas training ground that he had a "magnificent relationship" with Mourinho.