Leafs general manager Kyle Dubas announced the diagnosis on Wednesday morning ... saying the tumor was found while the 20-year-old was recovering from an injury he suffered at the beginning of this season.

"During the course of his recovery from this injury," Dubas said, "he developed some new, unrelated symptoms that required ongoing extensive investigations over the last few months."

Dubas says the prospect -- who plays in the KHL, one of the best pro hockey leagues in the world -- is now undergoing treatment at a German medical facility and he'll miss the rest of the 2021-2022 season.

"Our medical staff has been involved throughout the process alongside [Rodion's KHL team] and we are in direct contact with the facility on an ongoing basis to monitor his treatment and care," Dubas said.

"Rodion has the complete support of the Toronto Maple Leafs organization and we will continue to ensure he receives the best care possible throughout this process."

Amirov is in good spirits despite the news -- his agent, Dan Milstein, said he's working out every day and skating three times a week.

"I want to stay positive, and I want people to think positively about me," Amirov told Sportsnet.

"There are many other people that have their own sicknesses or illnesses. I want to show by example that I can give people hope."