Jaylon Smith was so good in coverage, that he all but took away many of the short routes in the middle. He also had amazing closing speed and anticipation.
To put it this way, even after he wrecked his knee, his instincts and anticipation were so good that the Cowboys took him the 2nd round.
In 2014 and 2015, we had enough trouble covering the WR's on the outside, that if anyone else had been playing at Smith's linebacker position, the TE's and WR's would have had an absolute field day over the middle at short range. Just look at 2016 for an example of that...
I'm also not exaggerating how bad Brian Van Gorder was when it came to playcalling.