Preciado had on Wednesday agreed to take the reins at relegated Villarreal and was scheduled to attend a news conference at the Madrigal on Friday.
"He was a man of football who represented the best values of his profession," Spain coach Vicente del Bosque told Marca Radio from Poland where his team are preparing to start the defence of their Euro 2012 title.
"It's almost impossible to believe so much could happen around one person in such a short space of time. He was a fighter."
"Life has hit me hard," he once said in an interview. "I could have weakened and ended up shooting myself or I could look to the heavens and carry on. I preferred the second option."
"Sporting convey their great sadness for the loss of someone who wrote their name in our history in golden letters and send their condolences to his family in these painful moments," the relegated club said.
He was Sporting's second longest-serving coach after having also worked with Racing Santander, Levante and Real Murcia among others.
Villarreal, who were hoping Preciado would lead them back in to La Liga next season after he successfully guided Levante and Sporting into the top division, said: "Villarreal wish to show their deep sorrow for the sad loss".
Mourinho ended up inviting Preciado to visit Real's training facility earlier this year.
"Manolo was always a worthy opponent," Mourinho said in a statement. "He had everything I like in people and sportsmen; character, transparency and the ability to fight against setbacks that were particularly cruel in his case.
"A great football figure leaves us and above all a very special person."
