Even if Mansour is back by Round 12, it will take quite a while before he will be back to his pre-injury playing ability. I've been told by people who work in the medical field that full recovery from an ACL injury is closer to 18 months for the joint to return to full strength and flexibility. As such Mansour would not be anywhere near ready in a conditioning sense for his knee to hold up under the added pace of an Origin game.
I only have to look at Jared Warea-Hargreaves when he came back in 2016 from his ACL suffered late in 2015. Whilst he was ok, he was not running with the same impact in many of the games he played in 2016 like he was prior to his injury, it was only right near the death of the season he started to look like his old self.
As such, I think TheFrog is right on this one. NSW won't risk him, not when they have quite a few options to choose from already.