Kell Brook of England will move up two weight classes to challenge Gennady Golovkin for his middleweight belts Sept. 10 in London/AP photo by Chris Carlson

Canelo Alvarez took a lot of heat when, as middleweight champion, he took on welterweight Amir Khan and knocked him out in the sixth round this past May. It’s only fair then that Gennady Golovkin absorb similar criticism, or so it would seem.

Tom Loeffler disagrees. Loeffler, who promotes Golovkin under the K2 Promotions banner, insists that Golovkin defending his title against welterweight champion Kell Brook on Sept. 10 in Brook’s native England (on HBO), is not the same.

“I think the fans realize it’s a completely different situation,” Loeffler told this newspaper this week. “That’s why tickets sold out as quickly as they did. I mean, literally, 11 minutes.”

Golovkin-Brook will be contested at O2 Arena in London. It seats 20,000.

Loeffler reminded a reporter that K2 Promotions tried to get a middleweight for Golovkin. Alvarez, rather than fight Golovkin this fall, gave up his middleweight belt and has moved back down to junior middleweight.

“Every middleweight that we tried to get in the ring with Gennady (turned down the fight), starting with Canelo and going to Billy Jo Saunders to try to unify the title, and then Eubank,” Loeffler said.

Loeffler thought he had a deal for Golovkin to defend against No. 2-ranked middleweight Chris Eubank Jr., but Eubank balked.

“Eubank was a long, drawn-out process,” Loeffler said. “So I think there’s a big difference there and the fans realize that. Look, if any one of those three we had been talking to had signed the contract, Gennady would be fighting a middleweight. But fighting an undefeated welterweight who is arguably the best in his division with a bigger heart than many of the middleweight champions, I think it’s a big statement.

“And the British fans realize how difficult it’s been for Kell Brook to get a big name as well. So it actually solves the problems of both fighters. It’s a good fight.”

Moreover, Khan was a small welterweight. Brook (36-0, 25 KOs) is a big welterweight. A source told this newspaper that when the muscle-bound Brook was on hand for a news conference July 14 in New York City, he was walking around at 178 pounds.

Golovkin (35-0, 32 KOs) never weighs that much between fights.