I think its fairly safe to say that if the medical staff thought there was a significant risk there, he wouldn't be playing. I wouldn't worry about it too much, these guys are professionals after all. Not only that, but there are a handful of cases in recent history of players playing with casts and being perfectly fine, Reggie White, Antonio Freeman, Nick Barnett, Cullen Jenkins, etc.
Certainly, if they thought there was a risk there, I would prefer him to sit out as well.
I also really doubt that the fact they are playing the rams factors into it at all, I mean I really hope it doesn't. The rams are an NFL team, and no matter how unlikely it is, GB could theoretically lose to them. When you get to the point of "oh we'll hold this guy out because its just the RAMS" I think you run into problems.