The Steelers have been through this situation before. Even if Le'Veon Bell misses extended time because of his Lis Franc injury, it is not the end of the world for the Steelers running game or Bell's career.
Look back to 2008 when Rashard Mendenhall missed all but 4 games of his rookie season thanks to a shoulder injury caused by Ravens LB Ray Lewis (which may or may not have been a bounty taken out on the rookie RB). Mendenhall then went on to have a few above-average productive seasons for the Steelers before once again succumbing to the injury bug and suffered a Torn ACL against the Cleveland Browns in 2011.
While Mendenhall's career isn't necessarily the best comparison to Bell's as of now (and other, less optimistic people could take negatives from it and "foresee" doom for Bell), it points out the fact that Bell's injury isn't cause for panic, contrary to the immediate reactions following the news during Monday night's game against the Washington Redskins. I know people were/are expecting Bell to be the Franchise RB for the Steelers and he could still be that guy. All I am asking is for some of you to have a little patience. It IS only Preseason after all. You need to take whatever happens right now with a grain of salt.