Billy Davis uses his NT on a majority of run based plays and as a gap stuffer. When he breaks it down for nickle/dime sets and pass rush situations he usually pulls his NT in favor of a DE/DT who can rush the pass and cause havoc. So typically, the NT is a 2-down type player.
The problem with using Terrance Knighton here is that he is a pass-rushing DT/NT and completely sucks against the run. The Jaguars benched him in favor of CJ Mosley this year during run situations. Knighton was only on the field during pass rush situations and eventually saw his total snaps slowly go down the entire year as Mosley performed better.
If your looking for a cheap free agent NT for the team to have as a vet presence, even if they are planning on drafting one. Look at Shaun Cody of the Texans. That is the type of NT who Billy Davis would like to have on his defense, as well as he can come cheap.
I would of put this as a fan shot, but most people don't even look at fanshots. I feel this needed to get out there instead of me constantly writing on peoples comments.