If he learns to watch the ball All the way into his hands before thinking about what he's going to do next he just might be...
We had a WR's Coach early on with Mallory who threw bricks at his Receivers during practice warmups... Needless to say, they learned to keep their eye on the ball,er..,brick...
Doubt if you could get away with that type of drill these days ((although the way I recall it the Receivers considered it sort of a badge of honor that they did it)) but it might be a good idea to give it a shot with Marshall... Right now he hovers between sorta of okay to almost good... With some serious Focus on bringing the ball into his hands he "could be" very good to Great...