Acting Geelong captain Joel Selwood was last night rendered unconscious and suffered concussion after accidental contact between him and St Kilda's Farren Ray.

Both players were going for the ball when Ray ran into him and made heavy contact with Selwood's head.

Selwood had to be stretchered off and was substituted by Darren Milburn, in accordance with a controversial new AFL rule which states a concussed player must be subbed out of the game.