Trainer Abel Sanchez says Gennady Golovkin faces a tougher task in Kell Brook than he would have done in Chris Eubank Jr.
WBA 'Super', IBF and WBC world middleweight champion Golovkin (35-0-KO32) had been set to face British champion Eubank Jr (23-1-KO18) but after talks broke down, IBF welterweight king Brook (36-0-KO25) signed up for a September 10 clash at The O2.
Although the Kazakh intends to fully unify the division eventually, Sanchez insists that the focus is on Brook and has stressed the reluctance of some middleweights to get in the ring with his charge.
In an interview reported on boxingscene.com, Sanchez said: "Eubank would not fight and demanded things that could not be done. [Once Brook came in], they just had to change the name and photo on the posters.
"Brook is a world champion. For me, the fight is more difficult than the one with Eubank Jr because we're going against a fighter who's never been beaten. He's never been knocked out or hurt like Amir Khan was with Canelo.
"We're facing Brook because there are very few fighters willing to fight Golovkin. They say yes, but want to wait and keep talking.
"Golovkin is 34 and needs to fight three or four times a year, so the negotiations had to be quick. This deal was done because Eddie Hearn is the promoter of both Brook and Eubank.
"The goal of Golovkin is to become the middleweight champion of the four organisations, but the title of the WBC was the most important. Now he'll continue making history and defending his WBC belt, the WBA and IBF, and in future he will win the WBO as well."
The WBO middleweight title is held by Britain's Billy Joe Saunders, who inflicted the sole defeat on Eubank Jr's record in November 2014.