The Seahawks view the third-down back as a very specific and important role, and all signs point to C.J. Prosise having that role. If Thomas Rawls and Alex Collins end up being 1-2 on the depth chart as the first- and second-down back, so to speak—though don’t count Christine Michael out in that battle—Prosise would still have a significant role in the offense in passing situations and in the hurry up offense, similar to how the Seahawks used Fred Jackson last season.
Jackson averaged 2.0 catches per game last season, so the Seahawks' third-down back isn't necessarily a role where fantasy owners can find a startable PPR back on the cheap.
SEA RB Fred Jackson - Week 15, 2015 Fantasy Football Outlook
Week 15 vs Cleveland
With Thomas Rawls injured, Fred Jackson had a season-high eight touches. The veteran could be in line for more although it seems Bryce Brown will be the first choice for Seattle. Jackson will most likely keep his role as the third down back. Even with a very good matchup, we don't envision Jackson as the primary back for the Seahawks. Keep him on the bench for Week 15.