Maybe Joe Vellano?
I'd seriously love to know what is going on behind the scenes with this guy. He just got cut, then resigned, then started a game, which would tend to indicate he's doing well then gets cut again. I haven't studied hi, but it appears like he has plenty of talent and is hustling, so maybe he's got an agreement to be the 2016 ping-pong ball and to be patient while he gets a few reps here and there and eventually sticks through attrition.
I can guarantee that we've had worse DLs taking up space at the bottom of the rotation. See: Bequette, Jake who got three years on the roster without really doing anything of note except to get drafted in the 3rd round. Bequette might actually be the least-useful player we've had in recent years when you consider he never played STs and barely got any snaps.
Then again, maybe Johnson is not Patriot material and is doing the same thing wrong over and over and not improving. When I rewatch the Steelers game I'll try and look for clues. Getting your ass kicked by David DeCastro is nothing to be ashamed of-- he does it to almost everybody he plays against.