"It was obvious on Thursday night he had done some serious damage to his neck ... on scanning today it shows a disk injury that is potentially quite a serious disk injury ... He is at serious risk of a catastrophic spinal injury," Halpin said.
Johns said he "was still in shock" but had told his family and his teammates and accepted that the writing was on the wall.
"They were sort of relieved I think, after a couple of injuries this year," Johns told reporters.
"I'm still in shock. I was in shock when it happened," an emotionally sapped Johns added.
A departure difficult
"Just totally numb. It was really difficult to tell my family.
"I made an emotional call to my brother Matthew and it was even tougher talking to the players this afternoon.
"I think the time's right."
He said he left the game with great memories and great friendships.
He said his teammates supported his decision when he announced it to them. "Each one came up and shock my hand."
Johns said he would have like to go out on a high with another premiership.
"I knew that the end would be coming soon, I knew it my heart that this year would be my last year.
"I would have loved to finish with a premiership. This could've happened when I was 19. I've achieved so much in the game, I've had some great memories and the main thing some great friendships," Johns said.
"I knew in my heart this year would be my last year but it's just unfortunate it's stopped five months before the end of the season.
"Two premierships definitely stand out. It was just so emotional winning both of those games. The biggest thing for me has been the friendships I've made which will last a lifetime ... It's been like an absolute dream."
He said he realised how lucky he was not to be in a wheelchair.
"I hate to think what a full-blown hit on the field could have done."
Blighted by injury
The latter stages of John's career has been marred with numerous injuries.
His first neck occurred in the 2003 season and he was forced to miss the last five matches because of a bulging disk.
Johns missed six weeks in 2006 after sustaining a broken jaw; in 2004 his season was ended in the third round with a torn anterior cruciate ligament in his knee; in 2002 a broken back ended his finals campaign; and in 2001 torn medial ligaments in his knee sidelined him for weeks.
|A near constant battle with injury |
His final season was a frustrating one.
He was knocked unconscious by a high tackle from Bulldogs forward Sonny Bill Williams in the fourth minute of their first-round clash and missed the second round while recovering from the concussion.
He returned against the Raiders, but the rust showed and he had a poor game.
A future immortal
Johns is likely to inducted into the Rugby League hall of fame as one of the game's immortals, the highest distinction a retired player can reach.
Johns was catapulted into stardom as he led Newcastle to premierships in a thrilling 1997 win over Manly and he lifted the title again in 2001 as the Knights upset the Parramatta Eels in 2001.
Johns played 23 State of Origin games for NSW and represented Australia in 21 Test matches, including the 1995 World Cup and the 2001 Kangaroos tour.
Johns ends his career as the greatest pointscorer in NRL history with 2176 points from 249 first grade games with the Knights.
He also won the coveted Dally M award three times and was the first player to win the Golden Boot award for the world's best player twice.
The Novocastrian also had a three match guest stint for English club Huddersfield in 2005.