Kassian collided with Gagner along the boards in the second period of the Oilers' 5-2 home exhibition win over Vancouver on Saturday. Kassian's stick struck Gagner's face, knocking out several of the Edmonton forward's teeth.
"As he nears the boards Kassian comes to a spinning stop, recklessly swinging his stick and striking Gagner in the face and breaking his jaw," said NHL director of player safety Brendan Shanahan. "While we accept Kassian's assertion that he did not intend to strike Gagner in the face, he is however responsible for the consequences of swinging his stick in this instance. In summary, this was high sticking."
Kassian received a double-minor for high-sticking on the play.
Gagner will miss the start of the regular season with the injury. The Oilers said he is out indefinitely.
The NHL also suspended Canucks' forward Dale Weise for Vancouver's final three pre-season games for an illegal check to the head of Edmonton forward Taylor Hall in the same game.
Weise's incident occurred early in the first period. He was assessed a minor penalty on the play for illegal check to the head.
"As Hall comes out of the corner with the puck Weise comes down and delivers a shoulder check directly to the head of Hall," said Shanahan. "Weise does not hit Hall squarely through the body and the head is the main point of contact."
Hall remained in the game and showed no apparent injury.
Shanahan also mentioned that Kassian and Weise's playing history were considered when deciding on disciplinary action. It is the first time either player has been suspended by the NHL.