The Eagles already had a good D when Wentz got hurt. If your D plays well enough, the backup QB just needs to be the "game manager" and not turn the ball over. They didn't need Foles to be as good as Wentz.

When A-Rod got hurt, MM knew that his D was mediocre for multiple reasons, so he still needed his QB to carry more of the load. When that didn't work out so well, and there were a lot of three and outs on the offense and inconsistent play by the D, that was a recipe for having the overall talent-level of the team exposed.

Fix the D this offseason. Priority #1.