I have Culter as my main QB and a decent range of WRs: Alshon Jeffery, Mike Wallace, Brandin Cooks and Kelvin Benjamin. I'm in a 14 team league and the best QB available on waivers is EJ Manuel; hence my hesitation to release Locker...
I think I probably would. Johnson should upgrade your starting lineup, or at least be a guy you can flip for someone else later, or allow you to flip Jeffery if Cutler goes down or something. You could do worse than Manuel, and both he and Locker are injury prone.
