Backup CBs who are returners typically aren't the best CBs. Lions shouldn't be wasting the 5th or 6th CB spont on returner unless he can actually play CB well. Slay, Mathis, Lawson,Bentley, so two spots remain.
I'll waste backup slot WR with Ross or 4th RB though for a return man. Ross contributed more than Logan ever did so moving up in the world a little. Keep Ross but give him competition and hope Ebron/Fuller develop is one option. Or draft a WR in Rounds 1-3. Round 1 if you want your eventual Calvin replacement ready by 2016/2017.