Preciado passed away at the age of 54, hours after being named Villarreal coach.
In his letter, Real Madrid coach Mourinho said: "Manolo was always an honourable opponent, who I got to know well when he came to visit us in March.
"He had everything that I like in a person and in sportsmen: character, openness and the courage to fight against blows. We have been left by a football figure and above all by a very special person. My memory of him will be heartfelt and permanent."
Preciado's former club Sporting Gijon, who were relegated in the recently-completed season along with Villarreal, said on their website: "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.''
National team coach Vicente Del Bosque, whose side are preparing for their Euro 2012 opener against Italy on Sunday, told Radio Marca: "We're shocked. We were having breakfast and it made a deep impression on us.
"He was a football man who represented the values of this sport. He was a fighter who made his mark in this world."
Spain midfielder Cesc Fabregas wrote on Twitter: "I can't believe the bad news I just woke up with. Hugs for the family of Manolo Preciado. RIP my friend."