The Panthers defense was just a mess. They switched to a 3-4 without the proper personnel and played McCoy and Poe out of position as 3-4 DEs. I watched quite a few Panthers games and it seemed their opponents would always run away from those two,. If they were in base, they'd run up the middle. The majority of the damage on the ground though was done when they were in nickel and Poe and McCoy would move inside together. The poor outside contain and gap filling of the DEs and LBs got abused in these situations on outside zones where Poe and McCoy were just out of the play by design. Panthers run D had a lot of issues, but putting any majority of blame on these 2 is wrong, IMO. It was a bad scheme with bad personnel.
Edit: They also rotated a lot, and often times had both Poe and McCoy off the field and their run D would get torn to shreds.