The 49ers just announced that DE Ray McDonald and S Dashon Goldson are both out for this Sunday's matchup with the Seahawks. Both are knee injuries and Maiocco made a good point in his post: If they're ruling these guys out now, clearly these aren't day-to-day injuries.
Hopefully they don't keep them out for more than this week. Obviously these are both guys from whom big things are expected. Considering the potential for a more aggressive defense, these are two guys who I'd imagine would see a bigger role.
Looking at the depth chart, I'd imagine guys like Keith Lewis, Tarell Brown and even Reggie Smith are guys who could see some more play in passing downs now. Mark Roman and Michael Lewis are both banged up with a groin and knee injury respectively, missing practice today. However, both are expected to play so we won't be too short at safety, thankfully.