Brook lost his IBF welterweight title last night by 11th round knockout as Spence Jr, fighting outside north America for the very first time, announced himself as one of the most dangerous fighters in the division.
The 27-year-old, who has drawn praise from Floyd Mayweather and Oscar de la Hoya for his ability, upset the partisan crowd at Bramall Lane with a flashy display of high-quality boxing.
Brook was knocked down early in the 10th but somehow made it through to the bell despite severe swelling around his left eye that was causing serious concern.
The Sheffield fighter explained afterwards that Spence Jr had caught him in the eye in the seventh round and left him struggling to see.
Kell Brook defeated by Errol Spence Jr Sun, May 28, 2017
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Kell Brook is defeated by Errol Spence Jr
And today he has told Sky Sports that he has broken his left eye socket, the same injury that he suffered in his last fight, a defeat to unbeaten middleweight champion Gennady Golovkin.
Brook also revealed that he was considering stepping up from welterweight after suffering badly later on in the fight thanks to having to slim down to 147lb.
And former world cruiserweight champion Johnny Nelson believes it would be a mistake to stay in the welterweight division.
"Kell will carry on, but to me it can't be at welterweight," Nelson said last night.
Kell Brook suffered an eye injury with which he persisted for four rounds
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Kell Brook struggles to see out of his left eye during the fight
"What he will do is go back, lick those wounds and then come back at light-middle. He'd be mad to come back at welterweight.
"Light-middleweight – or super-welterweight – that is where his strength lies.
"He wasn't as strong in the second half as he'd been in the first half and was manhandled and pushed back by Errol Spence.
"Errol said he'd found Kell so strong, so he knew he'd have to wait for the second half.
"A fighter who is tight and struggles at the weight is always weaker when it goes down the stretch and that's what happened.
"There was nothing in his tank, he was being bullied in the end."