In a surprise announcement that almost defies logic, Gennady Golovkin will defend his IBF, WBA and WBC middleweight titles against welterweight titleholder Kell Brook Sept. 10 at The 02 Arena in London on HBO’s World Championship Boxing, it was revealed on Friday, leaving many in disbelief. The bout will be fought at the middleweight limit.

The news comes as a shock since Golovkin (35-0, 32 knockouts) seemed to be far along in negotiations to face British boxer Chris Eubank Jr., who had already begun talking smack about what he would do to Golovkin in the ring. At the same time, Brook (36-0, 25 KOs) seemed to be firming up an agreement to face WBO welterweight titleholder Jessie Vargas later this year as well, but negotiations also seemingly stalled.

Instead, Golovkin and Brook will face each other in an improbable match-up reminiscent of Canelo Alvarez’s decision to take on the much smaller Amir Khan at a catch-weight of 155 pounds in May. Brook will retain his IBF weltwerweigh title while he takes on this mammoth assignment. The deal was finalized on Friday morning.

Tom Loeffler, who promotes Golovkin and celebrated a birthday on Thursday, acknowledged that pound-for-pound king Roman (Chocolatito) Gonzalez could also fight on Sept. 10 in a split HBO broadcast as the headliner of his own show from a different site. Gonzalez has been in talks to face junior bantamweight titleholder Carlos Cuadras. “If we can work out the details, that could still be a possibility,” Loeffler told RingTV.com. “He’d still be headlining his own show.”

Eubank seemed miffed that Golovkin is facing someone else. “I haven’t been shown one contract since my fight with GGG was announced,” Eubank wrote on Twitter. “I’m still ready & waiting to sign. Fighting Brook is a joke #2Small”.

Golovkin was initially frustrated when he learned he wouldn’t be facing the talkative Eubank, according to Loeffler. “But now he’s elated that he has a fight that’s set,” the promoter said. The idea of a catch-weight never came up in talks, he said. Desperate for a big-name opponent to face Golovkin after discussions with Canelo Alvarez broke down, Loeffler didn’t think twice when he was offered the chance to face Brook, despite the difference in size.

“He didn’t have to sell us,” Loeffler said of Eddie Hearn, who promotes Brook. “He’s the biggest name that’s been willing to get in the ring with Gennady. We tried to make the fight with Canelo. We tried to make the fight with (WBO titleholder) Billy Joe Saunders. We tried to make the fight with Eubank.”

Loeffler dismissed the difference in weights, referencing that Golovkin doesn’t usually blow up after the weigh-in, often fighting around 170 pounds. “Kell’s a big welterweight and Gennady is a small middleweight,” he said. “So when you see them standing side-by-side you’ll see. I think Kell is 5-9 and Gennady is 5-10 so there’s not going to be a big physical difference.”

Loeffler said a deal was nearly in place for Golovkin to face Eubank in London. However, last Wednesday, a number of “roadblocks” began to surface, leaving the fight in doubt. “It looked like everything was positive for the Eubank fight to happen,” he said in a phone interview. “We were excited to come over to the UK and fight in London at the 02 Arena and then last Wednesday it looked like the fight was not coming together. There were some road blocks of finalizing the last points on their side. Our side was always agreed to.”

At that point, Hearn mentioned to Loeffler that Brook was eager to face Golovkin and the discussions pivoted dramatically. Loeffler isn’t sure why Eubank suddenly got cold feet. He said that Brook was offered the same amount of money as Eubank while declining to reveal the figure.

“They were career-high paydays for both fighters,” Loeffler said. “I think it just comes down to if both fighters really want the fight it’s not that complicated to make a fight. If there’s hesitation on one side then, as we found out from many potential opponents to Gennady, then it becomes difficult to make the fight.”

Similarly, Hearn turned his attention away from the idea of Brook facing Vargas at a certain point. “I think it was just an issue when Eddie wasn’t able to close the fight with Eubank that’s when this opportunity came up to fight Gennady,” Loeffler said. “From what I understand, it’s a bigger upside for him and a much bigger fight overall.”

Loeffler applauded Brook for moving up in weight and said the fight satisfied goals for both fighters — to get the biggest name possible. “It actually solves both of their problems because Kell was having trouble getting opponents and so was Gennady,” Loeffler said. “It’s a huge event for both fighters.”

Brook said he didn’t hesitate when the fight with Golovkin was mentioned and believes he’s big enough to be able to hang with Golovkin. “Everyone knows I’m a huge welterweight and I know I will carry my speed and accuracy and even more power through the weights,” he said in a press release. “You have seen some huge names run from GGG but I’m running to him.”

Mitch Abramson is a former reporter for the New York Daily News and can be reached on Twitter at: @Mabramson13.