Well that would be a very bad assumption by any DC. So if you were to "take away " Diggs, it would require a double team a lot of the time. That would make Davis's, (which I'm sure you are aware is a pretty solid WR,) day a little easier. Add McKenzie's quickness along with how impressive he has looked in the slot, he would eat it up with all the attention on Diggs.
Kumerow I;ll give you, Although we know he can make plays too. In any event if that is any DC's mindset, I would not be happy with him if I'm HC. Also in no way Bills WR's would be "subpar" in that scenario.