If this was Major League Baseball, which has no salary cap, you bring him back. Hell sign him to a 1 or 2 year extension.
But this is the NFL. A salary cap league that makes you make tough decisions. Ask the Rams about giving an extension to Marshall Faulk after 2003. He played 1 more season, was injured in 2005, and the Rams payed for him for two or three seasons after he was done. You don't pay players in the NFL for what they have done. You pay them for what they can still do. Marshawn battled concerning injuries in 2014. He has a ton of touches on his 29 year old body and is walking that razor thin line of being able to play through those injuries and not being able to play through them.
I will once again say I have the utmost love and respect for Marshawn Lynch for what he has done for the Seattle Seahawks. I told my friends going into the 2007 NFL Draft I would rather draft Marshawn Lynch than Adrian Peterson. That is how much I think of him as a player. But you have to let a player go a year early than a year late.
I hate, Hate, HATE the New England Patriots. But they do this better than any other team. They got a first round pick out of trading Richard Seymour when Seymour was still a Pro Bowl player coming off an 8 sack season. He goes on to have 1 good season with the Raiders and the Patriots never missed a beat.
I hate to say Running Backs have become a dime a dozen. But every year RB's come out of the Undrafted Free Agent ranks, the later and middle rounds, to have good runs as a starter. Christine Michael and Robert Turbin are not even close to being on Lynch's level. But if you can get a RB in the 1st or 2nd round in 2015 that might be a slight step down from Lynch, and then take the money they save by letting Lynch go to get one of these outstanding free agent WR's (Demaryius Thomas, Dez Bryant, Randall Cobb, Jeremy Maclin, possibly Brandon Marshall, or to a lesser extent Torrey Smith) or TE (Julius Thomas or Jordan Cameron) It is wiser to invest money in younger players who you pay for what they can do, not what they have done.