Alexander is a back that you start any time he's healthy and A-Train is facing Minnesota, who was at the bottom of the barrel last year against the rush. I don't think you need to look much further. If you're not going to start Alexander on a regular basis, trade him for someone you will start, since you can probably get anyone other than Faulk or Green for him (along with another player or two).
And no, I'm not telling you to trade Alexander
I'm telling you to just start him unless he's going to be out or playing severly injured.