I don't think the problem is with Donald. He can easily fill the traditional role of a big, strong-side (TE plus RT) defensive end. But the Rams lack the right kind of athlete for the
other side in a traditional 3-4. That would typically be a speed-rushing, highly agile end who's bigger and taller than the outside backer behind him -- to go against the LT.
One could say that's Quinn. But it looks like he's moving to linebacker. Ironically, weight-wise, he's more suited to that anyway.
Instead, it appears the Rams will try to make do with a Donald clone opposite him: Easely. It may end up working just fine, though. But I wish they had that tall, agile, speed-rushing end of the traditional variety. At least to shake up the rotation. At least for that.