Kell Brook
Brook's trainer threw in the towel in the fifth round at the O2 Arena on Saturday. Getty

Kell Brook may have suffered his first professional defeat in his war with Gennady Golovkin on Saturday (10 September), but he left the O2 Arena with a reputation well and truly enhanced.

Against the most feared fighter on the planet, Brook didn't look out of place at 160lbs – having stepped up two weight divisions to a challenge a man so many in the boxing world have shirked.

Still the IBF welterweight champion, the Sheffield-born fighter has now opened up new doors for himself at middleweight and super-welterweight. IBTimes UK consider the options that could lie ahead of him.

Saul 'Canelo' Alvarez

Promoter Eddie Hearn described a showdown with Canelo as the 'natural' next fight for Brook. Canelo relinquished his WBC middleweight title earlier this year, a move many saw as an attempt to dodge mandatory challenger Golovkin. Canelo, whose only career defeat to date was to Floyd Mayweather Jnr in 2013, will instead take on Liverpool's Liam Smith on Saturday (17 September) in Texas. The timing of the fight couldn't be better; after collecting his bearings on Saturday night, Brook told the O2 Arena crowd he intended to fight the winner of Alvarez vs Smith.

A return to welterweight and Errol Spence Jnr

After his historic victory over Shawn Porter to take the belt in August 2014, Brook's three IBF title defences have been somewhat underwhelming. Against Jo Jo Dan, Frankie Gavin and Kevin Bizier, the Sheffield lad was largely untested and retained the strap at a canter. You would put your money on it being a very different story were Brook to defend against Spence Jnr.

Errol Spence Jnr
Spence has enjoyed a superb 2016 and is now the mandatory challenger for Broom's IBF title. Getty

The 26-year-old is one of boxing's fastest rising stars and his emphatic knockout victory over Leonard Bundu in August not only send a loud and clear message to the rest of the welterweight division, but it also made him the mandatory challenger for Brook's belt.

Brook has a choice to make; should he pursue fights beyond the 147lbs catchment, he will have to vacate his first world title sooner or later.

Chris Eubank Jnr

In the build-up to Brook's showdown with Golovkin, the Briton became embroiled in a war of words with the always outspoken Chris Eubank Jnr. Before the earth-shaking announcement of the fight, Eubank Jnr had been in talks with the Golovkin camp and felt he had been squeezed out. Eubank took exception and in an exchange with Golovkin on Twitter, he wrote: "After you are dong bullying Brook and you're ready to fight a real middleweight, I'll be there." Brook obviously took exception and advised the 26-year-old to sit back and pick up some tips from last Saturday's fight instead.

Eubank is a fine middleweight; if Brook is serious about consolidating himself at that level, he would be a compelling and demanding test. Like everyone else to have fallen to Golovkin, Brook was unable to stand up to his freakish punching power, but at times, did look big and strong enough to carry himself at 160lbs. Another fight at that weight would provide better clarification where his long-term future is.

Amir Khan

Like Brook, Khan took a commendable step up to middleweight earlier in 2016 to try and dethrone Canelo at the 155-pound catch weight, having previously fought above 140 pounds just four times in his career. After a bright start, however, the Briton was floored with one of the most devastating knockouts of 2016. In the aftermath of that fight, Khan floated the idea of returning to welterweight, but dropping down to super-welterweight could also be an option. 154lbs is also where Brook sees his future.

Amir Khan
Khan found middleweight to big a step to take earlier this year. Getty

Brook and Khan have circled each other for years and while 2016 has seen them veer off to pursue other fights, a long-awaited showdown between the two remains one of the most hotly anticipated all-British showdowns out there. Could a fight at 154lbs, neutral territory for two fighters who found middleweight a step too far for them, be a compromise?