As you can see, it looks like Reaves caught a rut in the ice as he was turning towards the boards which caused him to lose his balance. He collided with the boards very awkwardly and appeared to twist his left skate and knee. After getting to the bench, Reaves looked to had fallen while walking down the tunnel as well.
Being the or one of the toughest guys in the league, for Reaves to not be able to skate off the ice or walk down the tunnel under his own power means that he is hurt pretty bad. It would be surprising if he returns, but for now the Leafs will be playing the rest of the game with 11 forwards.