People have said the same things about Edelman, Amendola, Lewis, etc. With your statement, which is somewhat true, his role is so vital to how the D aligns, audibles, seeing things before the snap, etc, that you can't just expect to get it overnight via the draft or in FA. And, top of that, he just got the big contract.
Injuries are frustrating and it is part of the game and all teams have them.
If Hightower continues on with this, then yes, it will be disappointing and bad.
But, if you tell me your starting 4 LBS are Hightower, Van Noy (very underrated, IMO), Harrison and say Flowers or McCellin, you're in pretty good shape.
Van Noy is better up and around the line than in the Hightower middle of the field position.
I happen to think Flowers is pretty good and would be even better as our speed 3-4 MLB next to Hightower just as an example.
Maybe someone comes in and beats out Roberts in camp, but he wouldn't be a bad 5th LB.
Maybe the draft one fairly high with the idea Roberst would get bounced and to preapre for Harrison's departure past 2018, if they do resign Harrison, which I think they should. If he's genuine about getting deeper into the playbook here, they should be able to strike a deal. At the very least, he's a vet subrusher that we always seem to need at some point.
