As he confronted the probability that his young wife would soon lose her life, Guerrero prayed desperately that there was a guardian angel somewhere in the world willing and able to help her.
A bone marrow transplant was the only option left to save Casey Guerrero, who was battling leukaemia, from almost certain death. Doctors pegged the odds of survival at 50-50 should a donor be found in time.
It wasn't a sure thing, but it was hope, and the Guerreros needed all the good news they could get at that stage. Could a donor match be found? Would the operation be successful?
"We just had to have faith," Guerrero said.
A world champion boxer, Guerrero in 2010 surrendered his title and passed on a potentially career-defining bout in order to be at his wife's side as she battled for her life.
Guerrero was a rising star in 2010, an affable, charismatic guy who had speed, power, guile and plenty of courage in the ring. None of those talents, though, helped him in this fight. Robert's many gifts were useless in Casey's fight for her life.
But after he passed on that planned March 27, 2010 bout against Michael Katsidis, an anonymous donor was discovered in Europe whose marrow had nine of the 10 markers doctors studied.
In short order, the surgery was scheduled.
"Obviously, that was a time of great joy," Robert Guerrero said of learning of the donor match.
What was left unsaid was what would happen if the surgery did not work, if Casey's body rejected the marrow.
Fortunately for the Guerreros, they never had to confront that awful possibility. The surgery was a success and now, as Robert Guerrero is preparing for his welterweight debut Saturday in San Jose, California, against Selcuk Aydin, Casey is cancer free.
"Our prayers were answered," he said.
They were, except for one. Both Robert and Casey desperately wanted to meet the donor to, as Casey said, "say thank you for allowing me to live."
And so, earlier this year in Tucson, Arizona, Casey first met a 22-year-old German woman named Katharina Zech.
Zech was the donor whose bone marrow saved Casey's life. The resident of Munich wanted to meet the Guerreros nearly as much as they wanted to meet her. Since Zech had already planned her first trip to the US, the Guerreros flew to Tucson on April 14 to meet face-to-face.
When they saw each other, there was an instant connection. Their long embrace felt as if they were relatives seeing each other for the first time in years, not people meeting for the first time.
"It was really an amazing feeling," Zech said. "I wanted to give another person the ability to live. And when I met [Casey], it was like meeting my sister for the first time. We were not strangers."
Casey and Zech have become fast friends and will sit together at Robert's fight Saturday at the HP Pavilion in San Jose.
Robert now can compete and pursue the biggest fights free of the fear that he may have to cancel to care for his wife.
Zech's presence in San Jose this week is a sign to the devout Guerrero of God's presence in his life.
"I don't know the feelings Casey has, but I have to believe they're way different than mine, but for me, seeing them together and enjoying each other brings me a tremendous amount of joy and peace," Robert said. "I love my wife more than I can ever say and I have her here with me because of this woman. It was an awesome privilege to be able to meet her and to say thank you for what she's done for us.
"There's a bond between us that is always going to be there. She is the person who saved my wife's life. I will be forever grateful to her for that."