You agree to a $13.5 million deal because if you come back and get hurt again you are playing the following season on a vet minimum if you're lucky. Any agent in the world that advised Watson to not sign that, and just come back and play, should have his license revoked.
That the Packers offered it at all tells me that they think he's very likely to be able to make a recovery. He has the injury-plagued badge on his jersey, but the real nagging problem was soft tissue, which they did seem to get past last year. The ACL was an unfortunate one-off that can happen to anyone, and really isn't the injury it used to be.
Even if he went banana pants you're talking about what's probably going to be a handful of games. Teams would use it to drive his price down. This is really the best outcome for him, he gets to play next year with a nice AAV and earn a giant contract.