Souths chief executive Blake Solly and coach Wayne Bennett revealed the devastating diagnosis on Wednesday.
"There's goosebumps running through me now. It's just the last way you want someone to retire," an emotional Bennett said.
"But it is what it is and he's handling it great and he's helping us handle it a lot better as well."
Only 30, Arrow played 98 games for the Rabbitohs after Bennett lured the classy forward to the club in 2021, having handed the then-20-year-old his first-grade debut at Brisbane a decade ago.
Arrow also had four seasons at Gold Coast following his two-year stint at the Broncos and played 12 games for Queensland between 2018 and 2023.
He helped the Maroons win series in 2020, 2022 and 2023 while also playing in the Rabbitohs' 2021 grand final loss to Penrith.
"He's a wonderful young man to be part of the football club," Bennett said.
"We can't believe what's going to happen to him but he knows he's got the support of everybody in the change room and he knows he's got the support of everybody on the staff.
"And he deserves all the accolades and all the support that he can get because he has a battle on his hands. He knows that and he also knows that he won't have to do it by himself.
"He'll need that, but we'll always be there for him in this club. That's just the DNA of this club. He'll need us and we'll need him. No one's walking away from anybody."
The father of a one-year-old daughter is hugely respected in NRL circles and won the George Piggins Medal in 2025 as the Rabbitohs' player of the year.
He also won the Bob McCarthy club person of the year award in 2025, the Souths Cares Award for outstanding contributions to the community and The Burrow Appreciation Award in 2024.
Arrow was also a finalist for the NRL's Ken Stephen Medal in 2025 for his work with Souths Cares, Whatability and Vinnies Vans.
While declining to take questions, he bravely fronted Wednesday's press conference in Sydney as Solly read out a statement from Arrow.
"Thank you for the support I've received over what has been an incredibly difficult and uncertain period in my life," the statement said.
"After extensive medical testing and consultations regarding ongoing symptoms, I have recently received a diagnosis relating to a nerve and neurological condition.
"Further tests, specialist reviews and medical processes are still ongoing, and my doctors are continuing to assess my condition.
"Over recent months, my symptoms have affected different parts of my everyday life.
"On medical advice, I am not currently medically cleared to train or play at the required level, and I will be stepping away from those duties while I focus fully on my health, treatment, and rehabilitation."
Arrow thanked the entire South Sydney club - from fans, staff and teammates including Latrell Mitchell and Cody Walker, who were also present for the announcement - for their support.
He vowed to "fight this with everything I've got".
"What I need right now isn't sympathy or sadness. What I need is support, understanding and privacy while my family and I navigate this difficult time," he said.
"This is only part of my story and when the time is right I'll share more."
Arrow's diagnosis comes two and a half years after former Maroons forward Carl Webb died of MND aged 42.
The rugby league world was also rocked when Western Suburbs, Balmain and North Sydney great Scott Gale died of the debilitating condition at just 39 in 2004.
