Fantasy Upside
When you have Marshawn Lynch on your team, it's hard to get noticed as a running back. The Seahawks like Robert Turbin as the backup, so he has the confidence of the team if something happens to Lynch.
Fantasy Downside
Turbin has never carried the ball more than 80 times in a season and still doesn't have a rushing touchdown as he enters his fourth season. You need the ball to be productive and nobody in Seattle really carries the ball enough to be productive outside of Lynch.
Bottom Line
There isn't much value for Turbin unless something happens to Lynch. So if you have a big roster and you draft Lynch, Turbin would make a good handcuff. Otherwise, he just won't get enough touches to be relevant in standard fantasy leaguesuntil we see Lynch's decline.