Ryan Mason says he was "100% sure" he would return to football until just a few weeks ago, reports BBC Sport.
A recent CT scan showed the injury's effects were more serious than had previously been realised.
"That was a few weeks ago now and everything has really changed since then," Mason told the club website.
He added that until the scan he had been "been giving it my absolute all to get back on that football pitch" and had been "100% sure that that would happen".
But given the latest evidence, he explained the decision to retire was "a no brainer".
"I've got a son and a beautiful fiancée around me, so these are the sort of things that mean more to you than football when things are put into perspective," he said.
The former Tottenham man, who earned one England cap during his time at White Hart Lane, added: "The risks are just far too high - it's 100% the right decision and I'm not going to regret it."
The 26 year old continued: "I feel happy with the things that I have done. I have lived my dreams and I have achieved absolutely everything that I wanted to.
"I am excited about the future, I feel like I have still got a lot to give.
"It's such a unique position to be retiring at 26. I am still very young and I will be passionate about whatever I chose to do and I am confident that I will be a success in it because I know I will throw myself at it and it's going to be exciting.
"I am pretty sure that whatever road I will go down it will involve football because it's my life and I still love the game and I have always loved it."