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.