The Seahawks and the NFL confirmed that the Walter Thurmond suspension has begun this week, meaning he'll be out without pay for the next four weeks (per NFL rules on the Substance Abuse Policy, he can stick around at the VMAC facility, but cannot practice). Thurmond will be eligible for reactivation on December 23rd, Week 17, when the Seahawks face off against the Rams. The thinking that I've seen players mention is that the team wanted Thurmond to get his suspension over with as soon as possible, then have a chance to come back, recalibrate and re-acclimate to the game in prep for the Playoffs.
Taking his spot on the roster is free agent cornerback Perrish Cox, a 2010 fifth round pick by Denver. David Fucillo over at NinersNation provided this scouting report on Cox:
Over the last season and a half, Perrish Cox's primary role has been as the dime back, while also serving as Carlos Rogers' backup as the nickel slot corner. Rogers was healthy throughout Cox's time with the 49ers, and I don't recall any instance where Cox came on in the nickel. Cox also was listed as a backup returner, but he rarely saw time there as well.
The 49ers added Eric Wright in the offseason, and Cox was deemed expendable when the time came to activate Wright off the NFI list. My guess is the team simply felt Wright had greater upside. Assuming that was the case, the only options were to release Cox or rookie cornerback Darryl Morris. In special teams action, Morris had looked really sharp, so I imagine the team felt his upside was worth more than keeping Cox as a restricted free agent after the season. I don't know how getting waived impacts his RFA status after this season as he will only have three accrued seasons after this season.
The Cox signing points to Seattle's need on the inside at nickelback. Byron Maxwell, Jeremy Lane, and practice squader Deshawn Shead are all likely more suited for the outside, and thus Seattle must bring in outside (free-agent) players to fill a role on the inside. It's unclear what Antoine Winfield's status is at the moment, but he'd be another likely candidate if he's interested.