Rex is done before the start of the season if White has another performance playing against a 2nd team, perhaps 1st team, defense that is even remotely close to the one he had the other night .
There is a very valid reason for Rex to still be around during pre-season, he knows the offense is just part of it. He gives the 2nd and 3rd string players a guy who is capable of leading this offense without major hiccups. If you noticed the other night, he did horribly against the 1st string Titans defense after Cousins left, but by the end of the 2nd quarter he put together a very solid drive that led to a TD. That was with many of our 2nd string OL and younger receivers. That is valuable for a young player who is looking toward their QB to lead them. Not that Cousins couldn't and it appears maybe Pat White can, but we know what Rex can do...and he was solid enough in a few drives against lesser opponents giving the younger guys a chance to concentrate on their responsibilities and not whether their QB will **** up the play. Look at Geno Smith in NY for example of how a young 2nd string QB is not faring well and damaging the opportunity for some younger players to step up because he can't get the plays off correctly.
I know that sounds like a glowing compliment for Rex, but it's not. He's solid enough to give experience behind for the 2nd and 3rd string players against 2nd and 3rd string players. We didn't know that Pat White had that. We'll see how White performs with more time behind center against better than 3rd string players on a so-so team. Rex, while showing more Rexiness, does give a veteran presence to some younger players who may not have that chance with an unknown behind center like a Geno Smith while giving a proven commodity like Kirk Cousins the opportunity to sit.