As the 28-year-old's agent revealed, it all fell apart without warning.
"The deal with Valencia for Mariano was agreed," David Aranda told Radio MARCA.
"The contracts were done between the three parties and the player was excited, having understood that this was the best move for this season.
"But suddenly communication broke off [with Valencia].
"It was a big surprise to us, but we don't want to speculate as to why it happened. It just all stopped."
At the last minute Rayo Vallecano came in for the striker, but Aranda defended Mariano's decision not to take that option.
"Their president rang us and he did everything he could, we were talking until the last fifteen minutes of the window," Aranda explained.
"A number of clubs were interested in him when they found out Valencia were no longer involved.
"But this wasn't a decision we wanted to rush."
Mariano's contract with Real Madrid runs until June 2023, having joined the club back in 2018.
He has never been a first-team regular, but Aranda quashed any suggestions that Mariano is sitting on money by staying in the Spanish capital.
"Mariano has been earning under his worth at Real Madrid," he said.
"Every year he's had offers of a better wage.
"It's not about comfort, as this isn't his home.
"He's very competitive, but that's the way the market goes."
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