NE's behavior in prior A-Perp absences suggests that they feel that adjusting the approach is a better option than just substituting a backup Hernandez on the field. Can't really blame them, considering how much success they typically have.
Of course, that could be because they didn't have a quality backup, but the question still remains why you would have one of your highest cap numbers in a backup player. Or frankly, why a guy who is worth that much would want to sign here to be a backup. It isn't entirely out of the realm of possibility that NE did express some interest and Casey turned them down.
All told, I see no reason whatsoever to get on NE's case about this. WR, passing rushing DL and defensive backfield were much higher priorities than TE heading into the offseason and that is pretty much where all the investments went. The only real exception is Collins and I think we can all agree that a LB with coverage abilities was a bigger need than move TE as well.
So, all the prime capital went to positions of greater need and NE didn't even have the 4th, 5th or 6th rounders that might've been used on a higher profile backup option.