Manuel Preciado, who was appointed as Spanish side Villarreal's new coach on Wednesday, has died of a heart attack at the age of 54.
Preciado, formerly at Sporting Gijon, was due to be unveiled by the Yellow Submarine at a press conference on Friday.
A statement on the club's website read: "Coach Manuel Preciado died early this morning in Valencia and as a result of a heart attack.
"Villarreal wants to show its deep regret at the sad loss and wants to show their condolences to his family and all his relatives."
Preciado was in charge of Sporting Gijon for six years before departing for the Castellón club, who were relegated from the Primera Liga last season.
He bagan his managerial career in 1995 at Gimnastica before two spells at Racing B and Racing Santander, as well as stints at Levante and Murcia.
The Cantabria-born defender suffered two tragedies at the beginning of the century when, two years after his wife died from cancer in 2002, his 15-year old son was killed in a car crash.
Preciado was quoted as saying at the time: "I could have shot myself or I could have carried on".
On 2 April 2011, Preciado's Sporting side became the first team in nine years to defeat a Jose Mourinho team in a league match at home.
Mourinho entered the away team's dressing room to congratulate Preciado and his players, months after the Sporting boss labelled the Portuguese a "bad colleague" and a "scumbag" after the Madrid manager accused him of throwing a match against Barcelona five months previously.