People forget this, but he actually played well in 2016, including a couple of starts. Obviously the Atlanta game was disaster last year, but fans like to claim they had foresight that Green would be a disaster when it is actually hindsight they are relying on. His performance in 2016 suggested he was a very solid backup.
As for what this means for 2018 is anyone's guess. The Cowboys have to decide if the Atlanta game was an anomaly, although that's going to be hard because of how horribly bad it was, or if he is really the player he showed in 2016. If Green were to make the team, the good news is that Flemming will be the first option off the bench if needed at OT, and Green would only be an emergency in case 2 OTs got hurt.