I think there are a few layers to consider when it comes to Stevan Ridley.
First, I'd be shocked if they released him. He is a good runner working through a ball-security issue, which is similar to what Kevin Faulk faced early in his Patriots career. We saw him overcome it, so I don't think we're at an end-of-the-road type situation with Ridley.
I'd expect to see more of a rotation with Ridley, LeGarrette Blount (also a ball-security risk) and Brandon Bolden in that big-back role. The Patriots are deep at the position with those three, plus Shane Vereen, so I don't see any personnel additions.
Beat writer Mike Reiss said the days of Ridley seeing half the snaps are likely over, at least for the short term. It's unfortunately because Ridley has run well before fumbling and getting benched. For at least this week, Ridley can't be trusted in your lineup in what is likely an important game with playoff implications. In fact, of all the NE backs, Vereen is probably the only safe play here - at least until we see how this week vs. Houston plays out.