Rodion Amirov finished 15th overall in the NHL draft two years ago, and has since played for Salavat Ufa, a resident of the KHL.
At the start of this year's season, the forward suffered an injury.
During the subsequent rehabilitation period, a number of other, independent symptoms were discovered that forced him to further examinations.
Toronto now confirms that it has found a brain tumor.
"Rodion is currently being treated at a clinic in Germany and will not play again this season," said general manager Kyle Dubas in a statement.
- He has our full support and we will ensure that he receives the best possible care during this treatment.
Whether Amirov will be able to play again is highly unclear.
This season he has accounted for three points in ten games for Salavat Ufa.