The former New Zealand Warriors star - the team's all-time leading try scorer - has not played since 2017, leaving Salford Red Devils in Super League in 2018 after rupturing his Achilles.
Vatuvei moved into boxing but told reporters on Wednesday a scan before his first fight against David Letele in December, which he won by knockout, revealed the cyst.
"I've got a cyst in my brain, so that's been tough," Vatuvei said. "No symptoms, no nothing.
"Everyone had to do a scan before the fight and they found it then and tried to rule me out of the fight, but I went on with it and came out good.
"I had some fights lined up which were cancelled because of that."
The 33-year-old subsequently entered and won New Zealand television series Dancing with the Stars this year, but he is refusing to give up hope of returning to rugby.
"I haven't really announced my retirement or anything because I'm still in denial that I can still play," Vatuvei said.
Been an awesome show to be on and enjoying every moment with this lovely crew... I’ve survived 3weeks now and thanks everyone for your support... now back to practice for this week.. lesgooooooo #textMANU3333 #OUA pic.twitter.com/OEW0jZCfO8
— Manu Vatuvei© (@ManuVatuvei) April 30, 2019
"It's always been tough. I never watched a game when I came back from England, I never watched a game and I never went to a game or anything. I was missing it and getting itchy feet all the time.
"That's something I'm still trying to heal from. It was one of the hardest moments, but when I did this dancing it made it easier for me to get over it.
"I had to get over it somehow - it was a tough challenge for myself and I've enjoyed it."
Vatuvei has been helping Tonga prepare for an international against New Zealand on Saturday and added: "There's always a goal that I want to come back to the game.
"It all depends, I've still got some medical things that I've got to look at. Right now, my mind and my heart are telling me I can still play. But my body is saying no, so it's a constant battle at the moment.
"Right now, I can't do any boxing, but with footy they said because I must've had it a long time ago, I must have been all good playing.
"So I'm still deciding from there. Hopefully, I hear back from that soon and decide what my future holds."