Typically on nickel downs, you'd see 5 DB's and 2 LB's. It would've been Carr/Claiborne/Scandrick/Church/Wilcox, with Lee and Holloman, in all likelihood. Now I can see the extra coming in to play an underneath zone next to Holloman. Maybe if Matt Johnson actually stays healthy and plays well, he can play deep and allow Wilcox to fly around underneath in the Sean Lee coverage role.
Remember, even though it's a coverage role, part of coverage still is being physical within the 5-yard cushion and physically matching up at the catch point. You can't just throw 6 CB's out there. Size and strength help, even in coverage. We'd be losing some ability to jam and play physical at the route break & catch points with a DB subbing in for Lee, but it's probably the next best option. Carter progressing as a coverage guy would really be best case, and allow for both the athleticism and physical abilities, but it doesn't appear to be realistic.