I tend to agree with the PFF evaluation. When McDonald came back, I was expecting Campbell and Heacock were going to put together a defense that allowed McDonald to not have to take on OG/OT every play and the ability to play in space.
Heacock has had success with the 3-3-5 defense so I wouldn't have expected him to deviate from that formation to make McDonald shine. But I think he could have made some tweaks to create a defense that put more pressure on opposing QB's and force turnovers. It would have been interesting to see McDonald as a OLB in the 3-3-5 who rushed the passer on most passing downs or used as a spy (instead of Vance).
As an NFL player, he will be an OLB in a 3-4 defense. He'd be great in a Von Miller, Bobby Wagner, DeMarcus Ware, Greg Lloyd, Clay Matthews type role.