The Welshman showed his bravery with his willingness to fight on while he was being outclassed at the T-Mobile Arena, but amid his inability to defend himself from increasing punishment, referee Tony Weeks came to his rescue.
Having previously considered retirement following the first defeat of his career, by Sergey Kovalev in 2013, he may again examine his future.
He eventually followed that defeat by an unsuccessful foray into the cruiserweight division, and also appeared fortunate to win his title when Juergen Braehmer was forced to withdraw through injury when they fought last year.
Such a one-sided defeat limits his options, and at 30 he lacks the ability to challenge the elite at 175lbs that includes Kovalev, the great Andre Ward, and now new champion Jack.
The Swede made a fine start to his first fight in the division after stepping up from super-middleweight. This was a match-up that long appeared likely to be decided by either Cleverly’s superior work-rate and size or the more polished Jack’s jab, ring-IQ and combinations.
He had fought to an entertaining draw against James DeGale in January, and in 2015 also defeated George Groves, whose skill sets Cleverly has long lacked.
Badou Jack poses with his championship belts following his TKO victory against Nathan Cleverly following the fifth round at T-Mobile Arena.Reuters
Jack landed with two big rights and a left during the opening round, and also landed the more eye-catching combinations against Cleverly’s ineffective work-rate until he also responded with a big right.
As early as then and despite that right, the fight already looked Jack’s amid his superior variety and Cleverly’s long-term inability to adjust. The challenger responded to that in the second with another hurtful right hand and continued to dictate the range and pace of the action.
In the fourth after Jack swung and missed with a left hook, he landed with another and the Welshman’s resilience began to break down. He had succeeded with several further lefts and a right, appearing to break Cleverly’s nose as it began to heavily bleed.
By the conclusion of the fourth the Welshman was in survival mode, struggling amid an aggression and power Jack has rarely showed since fighting at world level.
Sensing a likely stoppage victory, the 33-year-old retained that aggression from the start of the fifth, which swiftly became uncomfortable to watch. Cleverly could do little to resist the repeated, hurtful punches that were coming his way, and after two minutes and 47 seconds, when trainer Darren Wilson should have withdrawn him, referee Weeks intervened.
“Jack was very strong,” Cleverly said.
” He caught me and broke my nose. It was a downward spiral from there. I was wounded and protecting myself.”
“I thought the stoppage was premature. He had just swung and missed but it’s up to the referee in the end. I’ll re-evaluate. I have a lot of miles left in me. I’m a two-time world champion. I have to take it from here and decide where I go.”
Jack said: “You can’t leave it in the hands of the judges. You’ve got to go for the kill. I think Adonis Stevenson, let’s get it on. Unless you want a trilogy with Andrzej Fonfara, let’s get it on.”