Let's compare Philly Brown's numbers to Devin's.
20% of Devin's total catches (9/44) and 20% of his total yards (135/660) came in the last 6 games.
40% of Philly's total catches (26/63) and 33% of his total yds (251/771) came in the last 6 games.
You can see the drop off in production for both, but the drop of for Devin is much greater. This is where I think route running, separating from DB's, etc, come into play. I think this is basically what Byrnes was saying when he said Devin hasn't proven much outside of his ability to run in a straight line really, really fast.