It is an interesting idea but I think you should hold off for now. Freeman will be playing soon. With Bell he could be out a lot longer and who knows who he'll be playing for. It is a lottery ticket for sure but Freeman is the safer play.
I don't know which side I would prefer. A lot depends on what else I have. You have three startable RB's so you don't need someone right now (sure a healthy Freeman and an in camp Bell are upgrades).
I don't like the knee issue as I don't think it will go away and may hamper him all year so he won't be the guy you expect. It could also lead to partial games and questionable tags and just more touches for Coleman. I think I lean Bell. Sure he may not be back until week 10 but when he comes back you will know how he will be used. Sometimes that "is he in or out" question is too much to deal with it. I think I lean Bell.