Per NFL Network's Tom Pelissero, Lynch's personal coach, Tareq Azim, said the five-time Pro Bowler began working out "in secret a couple of weeks ago" after visiting the Seahawks facilities.
Azim noted Lynch "went 'hood' on preparation" and had 16 training sessions over eight days to get ready in case he was needed.
Lynch's agent announced on Monday night his client agreed to a deal with the Seahawks for the remainder of the season, per ESPN's Brady Henderson.
Seattle's stable of running backs was decimated during Sunday's 27-13 loss to the Arizona Cardinals.
Head coach Pete Carroll told ESPN 710's Danny and Gallant show that C.J. Prosise broke his arm and Chris Carson fractured his left hip.
Carroll also confirmed during the interview that Lynch was coming in to meet with the team:
“I'm fired up for it. He's got to get through hoops he has to jump through here, physical and stuff like that, but he's been working really hard. He's really excited about the chance to do something, helping out. I think it's… great if he can get out there and tote the ball for us. The circumstances rolled just at this time. He could have four, five games left in him. Maybe that's just what we need."
Lynch spent six seasons with the Seahawks from 2010-15 after being acquired in a midseason trade with the Buffalo Bills. The 33-year-old was one of the NFL's best running backs during that stretch with four straight seasons of at least 1,200 rushing yards and at least 11 touchdowns from 2011-14.
Following a one-year retirement, Lynch played two seasons with the Oakland Raiders from 2017-18. His last appearance in an NFL game was on Oct. 14, 2018, in a 27-3 loss to the Seahawks.
Seattle has a huge home game in Week 17 against the San Francisco 49ers that will determine the winner of the NFC West.