Ball will most likely go on IR. I think Knight is expendable because he's behind both Nsekhe and Steele as an OT backup and behind McGovern as a guard backup. But you may be right that Farniok plays a role in making him expendable, by earning a roster spot as backup center.
We're going to keep nine or 10 linemen. We had Smith, Williams, Biadasz, Martin and Collins as the five starters. Nsekhe and McGovern as the top two backups. If we keep Steele and Farniok, that puts us at nine. Then, Ball initially takes a spot before being put on IR. We could go 11 initially, then drop to 10 if we keep Knight, but I'm not sure we want to carry 10, especially if Ball is only temporarily on IR and can be brought back.
I like Knight as a guard/right tackle better than you apparently do, but it makes sense to swap him as a backup for a backup that we need more.