As Loyal says, every little bit helps, and some people here consider Gustin to be their best LB. On the other hand, LB is an area where SC has depth, so I don't expect a huge drop-off.
I suspect that the major impact on his replacement will be in terms of recognizing our formations and adapting their coverages. In short, experience and familiarity with gametime, on field conditions. I expect us to run an offense that forces them to stop the pass. They will probably rush 3 and drop 8, and the question then will be whether we can run/shovel pass our way out of that, and how much time the line can give our QB against a decent 3 man rush. Having a more mobile QB may turn out to be the biggest factor in that...time will tell.