The Patriots underwent a youth movement at the CB position in 2015, letting veterans Darrelle Revis, Brandon Browner, and Kyle Arrington go and replacing them with Malcolm Butler, Logan Ryan, and Justin Coleman. The Patriots also have another young CB prospect in Darryl Roberts, but it's not a smart idea to rely on him as the answer. The Patriots are good at the top of the depth chart, but could use some more depth for Special Teams and in case injuries strike the position again. My preferred method would be to sign a veteran to the roster to compete for that last spot on the roster with a late round draft pick/UDFA.
Harlan Miller: Miller comes from a small school program in Southeast Louisiana State that previously sent Robert Alford to the NFL in 2013. Miller comes in at 6'0" 182 with 30 7/8" arms and 71 3/8" wingspan, so his ability to run and change directions will be more important. Miller has had a strong week so far in the Senior Bowl and could make himself a Day 2 pick in the draft.
Maurice Canady: Canaday is a talented, yet inconsistent player that could help the Patriots secondary. Canady is 6'1" with 31" arms and 75" wingspan. He has a bit more size and length than what the Patriots secondary currently features, but the inconsistency will keep him from being drafted before the 3rd round. Coaching isn't an issue for the Patriots at the CB position given that they've built a strong CB group with a 3rd rd pick and two UDFAs, one of whom had to literally try out for a roster spot before making the greatest INT in Patriots history 8 1/2 months later. It will be interesting to see if Canaday returns to his 2014 form with better coaching.
Deiondre Hall: If you're looking for a big and lanky CB, you've found your guy. Hall measures in at 6'2" with 34 3/4" arms and a 82 3/8" wingspan. The Patriots had issues covering big receivers in 2015, particularly against Brandon Marshall of the Jets. Hall could establish himself as a 2nd round pick if he can run well and show that's he able to change directions fluidly unlike most CBs of his stature. They probably shouldn't match him up with the Julian Edelmans of the world.
Darian Thompson: The Patriots have depth at the box safety with Patrick Chung and Jordan Richards both under contract through the 2017 season. However, their depth at the deep safety position could take a hit if Duron Harmon walks in free agency after this season. Thompson would likely have to be taken with the Patriots top pick if he's even still available that late. The Patriots should keep their options open at the deep safety position in 2016 because the Patriots don't want to be an injury to Devin McCourty away from "Oh fudge!".