It's really just as you said. Slaton gets his chance to day against a very tough defense. But if he performs, expect that lineup to continue even if/when Ahman returns. If he struggles, it will be a RBBC including any healthy body.
As a Slaton owner I hope he shows something this week. It will be pretty tough going with the Titans D in front of him today. Still think he will get more chances as the years of pounding at RB is starting to show up on Green. Green can only hope to be a healthy for a small string of games, so Slaton should have some chances later in the season if he doesn't get a solid one this time around.