via Sean Farrell at NHL.com,

Edmonton Oilers left wing Taylor Hall left his team's game Saturday against the Ottawa Senators in the second period with an injury to his left knee. He will not return.

Hall was called for slashing after he was hit hard by Senators defenseman Eric Gryba at 11:04 of the period. He came out of the penalty box and stayed on the ice briefly before going to the dressing room with 6:42 left in the period.

Watch the play below...

added 6:09pm, from David Staples of The Cult of Hockey,

In the second period of the Ottawa Senators vs. Edmonton Oilers game on Saturday, Senators defenceman Eric Gryba knocked Taylor Hall out of the game with a knee-on-knee hit. Hall left the game with a left knee injury.

continued with picture evidence... and I have also added a 2nd video below, scroll to the :40 mark to see the knee-on-knee hit...