When the Cards fired Arians and replaced him with Wilks this season, they also replaced DC James Bettcher (a 3-4 guy) with Al Holcomb (a 4-3 guy) which disrupted the roles of several existing front-seven players.
Riddick played almost exclusively on ST for the first four games of 2018. Since his "promotion" in wk-5, Riddick has played 100% of the Cards' D-snaps and, in just four games, has pretty much matched his production from his entire rookie season in 2017. He has 28 tackles, 2 sacks, 3 QBH, 4 PDs and 1 FF. He also dirt cheap on only the second year of his rookie deal. I doubt that Keim would be eager to part with him now (although I'm not sure he's the brightest bulb on the tree) .
In contrast, Bucannon - who had averaged about 76% of the D-snaps during the first four years of his career as a SS/LB hybrid in the 3-4 (and sparingly on ST) - is suddenly being used only intermittently on defense (35% of D-snaps) and extensively on ST (56% versus 13% during his first four seasons). Pretty clearly, he's been a victim of the defensive scheme change.
He may well be expendable from Keim's perspective -- especially since his 5th-year option salary (picked up a year before the coaching/scheme change) is a fully-guaranteed $8.72M. If the Pats traded for Bucannon, his remaining 2018 salary would hit the Pats' cap for about $4.6M -- a couple million too much, as things currently stand with cap space. If Keim was willing to eat that couple million by converting salary to bonus, the trade might fly.
I'd guess that the asking price for Bucannon's proven abilities might be as high as a 2nd-rounder, especially if Keim has to eat some cap to move him. The floor might be a 2019 3rd-rounder. Bucannon is UFA at the end of 2018. If other teams view him as primarily an LB, he could easily receive a contract large enough to give the Cards a 3rd-round Comp pick in 2020. So, a pick traded for him now would probably need to exceed that value.
Another potential downside for the Pats is that Bucannon could end being a half-season rental for $2.6M by pricing himself outside the Pats range in the 2019 FA market.
