Yeah, he's average at best. He's rarely around the deep ball on passing plays. He's rarely even in the screen, within 5 yards. That means he doesn't have the ability (or is poorly coached), because he never can even guess the deep route and be all over it.
Sure, he ran down Barkley, but we need WAYYYY more out of him against Seattle, since Russell likes to throw deep. If Heath doesn't have a big game, I don't think the Cowboys defense will be all that effective against the pass, unless the entire Dee line (not just D Law) plays out of their minds. And they should have consistent success against that hobbled Seahawk line.