A potentially fatal brain aneurysm found only a month before he was due to fight on his big brother's world title undercard in Cardiff means John Parker has a big decision to make.

Should he wish to continue his professional boxing career, John, who at 24 is two years younger than Joseph, will need surgery to repair the problem found in a routine pre-fight MRI scan at the start of the year.

A brain aneurysm is a bulge in a blood vessel in the brain, which is often symptom free. But should it rupture, it can be deadly.

It meant John couldn't fight at the Principality Stadium before Joseph took on Anthony Joshua in his world unification bout and has put in doubt his future in the sport.

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It was a tumultuous few weeks in the lead-up to the biggest fight of Joseph's career; the quietly-spoken John had to process the diagnosis while being mindful of not causing any distractions to his brother. He is still coming to terms with it but is back training again for fitness as he weighs up whether or not to go under the knife.

"I was okay when I was told but as soon as Joe called me and asked, 'uce [brother], are you all right?', I was overwhelmed and started to get teary," John said in London this week in the lead-up to Joseph's fight against Dillian Whyte at the O2 Arena tomorrow morning. "I tried to hold it in."

John will be in Joseph's corner tomorrow, just like he was in Cardiff and many fights before that.

"It's small enough that I can do everything on a daily basis without a problem, but if I want to box again, which I haven't decided just yet, I will most likely have to have an operation," John said of the aneurysm. "It's between two operations; one is where they drill a hole through my temple and they clip it for good or the other option is they can thread a wire through my groin and they clamp it, which they will have to do every so often.

Joseph Parker with his younger brother John during training at The Wreck Room in Auckland in May. Photo / Photosport
Joseph Parker with his younger brother John during training at The Wreck Room in Auckland in May. Photo / Photosport

"The neurosurgeon I saw was confident he could do a good job but there is always that risk [but] the last thing I want is to be paralysed or something and to be a burden on anyone.

"My mind just came right - yesterday I got back into training. I'm lucky I can lose weight quite quickly. I want to get back into shape, so when it comes to operate or not, I'll be healthy and fit and will react well to the procedures."

He added of his reaction when told of the diagnosis: "I said 'can I still fight?' and they said 'no, no, no, this is serious'."

John, who divides his time between Auckland and Christchurch, where he trains with Bryan Barry, the brother of Joseph's trainer Kevin, was a talented amateur boxer who won national titles and Golden Gloves awards at middle-weight and light heavyweight.

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He has fought three times for three wins as a professional - all on Joseph's undercards.

He is now at a crossroads - not only in terms of his professional boxing career, which he knows will never be as successful as Joseph's, but also his life. He is thinking about taking up a building job in Christchurch.

Heavyweight contenders Dillian Whyte and Joseph Parker meet during a press conference to announce their clash at London's O2 arena. Photo / Getty Images
Heavyweight contenders Dillian Whyte and Joseph Parker meet during a press conference to announce their clash at London's O2 arena. Photo / Getty Images

"Joe and I are pretty close. In a sense, I'm living my dream through him. Growing up, we've always shared the passion for boxing. 'We'll be better than the Klitschko brothers, we'll be champions of different division ... we'll conquer the world'. Looking back on it now, at least he got to do it."

John said he yelled his lungs out at the Principality Stadium when Joseph lost a 12-rounder by unanimous decision, the first time Joshua had been taken the distance in his professional career. He will do the same after walking out behind two of his uncles and Joseph in front of what will be a partisan crowd of 18,000 at the O2 Arena. "He has always been in my corner growing up and I've always been in his," John said.

As for the result in a fight which could make or break Joseph's career?

"I made a prediction earlier in the week and I'm sticking by it. It's goodbye to Dillian Whyte in round three."