Flynn's performance against Detroit last year seems like reason for optimism.
My expectations for Harrell would be measured but I'll bet he can orchestrate a couple drives against most NFL defenses. He might struggle with third-and-long (i.e., the offense might stall) but he should be able to deliver those short, quick passes to receivers on slant routes that keep the sticks moving. It's never been easier to pass across the middle than in today's NFL.
We're not built to lean on the run, so I hope that conservative play calling and quality receivers will help Harrell carry the load.
All of this is speculative of course, reflecting my confidence in the Packer organization to develop a competent backup QB that will execute well enough to give the team a puncher's chance...