I believe the answer to your question is Barrington. I know they stuck Lattimore in there for a while once Brad Jones (Mack) was hurt and then eventually Barrington took over, but I still think the answer is Barrington.

Looking at the roster, we know that Bradford cannot be the Buck, so the immediate need in terms of bodies seems to be on the bigger role. But, getting a starter at the pursuit role would free up Matthews to play on the edge or on a fucking cloud if they wanted. Either would help the depth on the squad. Presuming a buck is taken, there's a chance that person could unseat Barrington as a starter, so that could be an upgrade, but taking the smaller role ILB would potentially upgrade our pass rush AND coverage.