If it was ME - and I realize Kubiak and I aren't the same person, I would -
Start TJ Yates this Sunday, unless he just plays so awful at practice that my hand is forced. My reasoning is, I'd rather put a player out there with some NFL experience under his belt out there in the middle of Arrowhead against an unbeaten team. To throw Keenum into the ring in that situation would be to almost want him to fail. That's a tough spot for a dude who's never started an NFL game.
If Yates plays god awful, I keep the competition open, and use the extra week we get at the bye to further evaluate. If Keenum looks better than Yates in practice (or if they both practice well) I go with Case, for the SNF game against the Colts. I'd much rather his first game be at home, where the crowd is behind him.
If Keenum sucks, we can trade for James Casey, or run the wildcat with Cushing.