My thought is they probably had Luke set as the backup if Vederal got hurt, but they weren't planning on a helmet coming off. In that case they only have 25 seconds to get someone in the game, call a play, and run it. Not a lot of time to talk about ramifications of putting Luke in even if it's only for one play. Could have called a timeout, but in a close game you hate to use them if you don't have to.