The injury game is always a tough one but if you are one who is willing to go that route then James Connor seems like a must have. I would prefer him to Breida because Bell gets hurt just as much as Hyde (although people kill Hyde for it and give Bell a pass for some reason) and I think Tomlin just loves to use one back as much, or more, than any coach in the league.
I love Shannahan RBs but he does rotate them more than Tomlin IMO and if Hyde goes down I anticipate a quick signing of Tim Hightower, which muddies the situation just a tad.
ETA: Pittsburgh also has a much better offensive line...much better team overall really, with or without Bell, that should afford more opportunities for their starting RBs.