The next 4 games are against the Hawks, Niners, Jags, and Texans. Niners are probably the easiest defense out of that group, sure, but it doesn't make any more sense running Rivers out against the other three than it does us. Jags defense is good, obviously the Texans defense is great, and they are both in their division. None of those games are gimmes that Rivers is going to tip the odds in the Colts favor.
They need to try and steal a win or two somewhere in that string of games. If they are going to use him, it makes the most sense to get him out there as quickly as possible and hope he has enough juice to help them sneak into the playoffs, because losing even one more game will make it extremely unlikely they make it in.