The New England Patriots need to upgrade their running game, there is no way around it. Some of the struggles in 2015 had to do with inconsistent offensive line play, but the running backs themselves - outside of Dion Lewis, who right now is recovering from a torn ACL - need to improve as well.
With numerous big-name free agents like Lamar Miller, Matt Forte or Chris Ivory already off the board, the Patriots need to look somewhere else. Apparently, "somewhere else" is Denver, Colorado: according to Broncos beat writer Mike Klis, New England is one of four teams negotiating with restricted free agent runner C.J. Anderson, trying to lure him away from the defending world champions.
The Broncos have tendered the 25-year old Anderson with the lowest tender: right of first refusal. This means that while Denver can match any incoming offer, the team won't get any compensation in case Anderson leaves the team to sign with someone else.
New England has some familiarity with trying to sign restricted free agents: in 2013, the team tried to get Emmanuel Sanders off the Pittsburgh Steelers but the wide receiver eventually stayed in Pittsburgh, only to sign with - coincidentally - the Denver Broncos the following offseason.