Reports on both Zay and Matthews are that they are smart and play smart, and hearing them in the interviews reinforces their command of the game, especially how impressive Matthews was in his breakdown of the Bills offense only one week in. The same can be said of Boldin's one week stint and the time he put into learning it.
So, yeah everyone can soft pedal about Belichick's preference for certain kind of players, but God forbid anyone should comment that McD prefers players who seem smarter than your average WR.