Buy. He will start. The amount of time he stays in the game will depend upon both his ankle and the team's performance. I wouldn't be surprised if the entire substitute offense came in for a series or two in the first quarter, as they did against USM.
One of the problems of having a Heisman candidate is that you need to give him a chance to run up some numbers. I have heard head coaches discuss this in the past, that the team focus suddenly becomes helping one player pad is stats. It is part of the reason Alabama did not have a Heisman winner under Bryant: He got John David Crow a Heisman at A&M, and he really didn't want to go through that with another player.
I think Mullen and Prescott will keep some perspective, though. Get Prescott some reps, but do what is best for his ankle and the team in the long run. The Heisman voters are not going to abandon Dak just because he coasts through this one.