Having a good run defense isn't too difficult. There are many things a DC can do to help out stopping the run. Clearly you can play more men inside the box, play bigger alignments, etc. However, to have an effective run defense cannot be undisciplined, beaten up at the point of attack and tackle poorly.
Yes, we all know there are personnel issues, especially inside at the DT position, but the Edge lineman, including DLaw have struggled against the outside zone runs for years. Our linebackers have really struggled to get off blocks, sure better DT's could help, but they need to be a better job of reading and reacting before a hat gets on them.
Jerry/Stephen have to give Zimmer the necessary resources if he doesn't have the right players to run his scheme. If some veterans continue to struggle with the fundamentals, Zimmer will not put up with it, like Dan Quinn did for years.
Zimmer has a strong track record of having good, disciplined defenses, we have to trust he will find some solutions to least make the run defense hold up.