Eddie Hearn, promoter for WBA "regular" super middleweight champion Rocky Fielding, was shocked when he received a phone call from Golden Boy Promotions, to ask him if his fighter was interested in facing Mexican superstar Canelo Alvarez.
A deal was eventually finalized for December 15 at New York's Madison Square Garden.
The fight will be Alvarez's first since his September victory over Gennady Golovkin for the WBA, WBC middleweight world titles.
When the fight was announced, fans were in shock that Canelo was heading up to 168-pounds to face Fielding.
"It's something I didn't think I'd ever say," Hearn told Sky Sports News.
"Rocky Fielding will defend his WBA title against Canelo Alvarez at Madison Square Garden. The Scousers are coming - this is going to be the trip of a lifetime.
"I got a text message from Eric Gomez and thought, 'this could be interesting'. He said 'what's Rocky Fielding up to?', I said 'he's looking for a defence' and he says, 'well does he want to fight Canelo?'. I nearly dropped the phone and said, 'possibly, we are looking at our options' (laughing).
"I spoke to Rocky - and you have to get the best deal for your fighter - but this is the opportunity of a lifetime, this is a Cinderella story. You couldn't make this up. This is a fight that will change his life forever. He will go in as the much bigger man, a big underdog in the fight. We know that Canelo is coming up from 154 pounds to 160. Rocky Fielding is going to be 180 pounds in the ring."
The 28-year-old Mexican will take a 50-1-2 record with 34 knockouts into the fight. Fielding, 31, is on a six-fight winning streak that improved his record to 27-1 with 15 knockouts.
He won the WBA super middleweight title with a technical knockout of German Tyron Zeuge in July.