ESPN Senior NFL Insider Adam Schefter tweeted Monday night that the deal is for this year only which is the last regular-season game and the postseason.
Seattle is in desperate need of running backs after Chris Carson and C.J. Prosise both suffered season-ending injuries in Sunday's 27-13 loss to Arizona.
Coach Pete Carroll said earlier that reports are Lynch is in good shape.
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Lynch has not played in more than a year. His last game was Week 6 of last season with the Oakland Raiders.
Lynch spent parts of six seasons with the Seahawks before spending the 2017-18 seasons playing for the Oakland Raiders.
Just over a week ago the Oakland native was handing out tequila shots at the tailgate for the Raiders last home game, according to fans on Twitter with photo evidence.
The Associated Press contributed to this article.